It was nothing short of a mystery how they managed to stay close enough on the scoreboard to earn a losing bonus point but succeed they did.
The last-minute try by Fergus McFadden resulted, it must be acknowledged, from a moment's carelessness by Clermont, who had turned off to indulge in premature celebration, but how clued-in were McFadden and architect-in-chief Jonathan Sexton to be in the position to take advantage.
Leinster can look upon this late bonus as the Christmas gift beyond all gifts for it keeps alive, if only just, their wafer-thin hopes of advancement.
Clermont were immense in every aspect of the game. They came as men on a mission and they effected that mission with a professionalism that reflected all of the quality we have come to recognise and admire over the years in the best of French rugby.
Leinster were dominated from the opening whistle and the manner in which Clermont stepped up their game from the previous week and the way, especially, in which they negated the threat that Leinster had posed in the first match spoke volumes for the skill and application they brought to the Aviva, and were a tribute also to the clever work of their coaching/management team.
The enduring truth about any rugby game is, however, that performance is dictated by the work of the front eight. They decide who enjoys majority possession and they define the quality of that possession. In these regards, Clermont were superb.
Leinster's tighthead Mike Ross was turned inside out by Clermont loosehead Raphael Chaume and with Davit Zirakashvili performing similar acts of savagery on Heinke van der Merwe on the far side, Leinster's primary set-piece platform was in disarray from the off.
This prevented the defending champions from establishing any sort of foothold in the contest and, as captain Leo Cullen admitted afterwards, Clermont dominated in every facet.
Clermont's rugged pack was particularly effective and lorded it in every phase.
The way they committed to the breakdown and the controlled aggression they brought to bear meant that Leinster were always under pressure and turned over ball too often to hope to survive.
Former Ireland coach Eddie O'Sullivan coined the phrase 'mad dogs let loose in a meat house' during his time with the national side and that perfectly sums up Clermont's attitude. They scrapped for absolutely everything and when they hit the breakdown they left very little behind them.
Indeed, so dominant were Clermont the surprise was that they did not score more than one try. But this deserves to be regarded as a measure of achievement for Leinster who were forced to spend so much time in their own '22'.
In defence, Andrew Goodman, in particular, put in a massive shift. His tackle count was in the double digits and in this endeavour the likes of Jamie Heaslip and Sean Cronin were able lieutenants as they stood resolutely in front of the rampaging French.
They tackled and covered and tackled and covered again to keep at bay a strike force of formidable strength and speed.
But rugby is, when it is boiled down, a game of numbers and eventually the Leinster defence was breached. When Wesley Fofana crossed for the first try of the game before half-time there was a sense of inevitability about it. You can only hope to keep an attacking side like Clermont at bay for so long.
It was a mystery at times why Clermont didn't go for the jugular more ruthlessly.
In the second half, for example, there was surely a penalty try on the way after the Leinster scrum coughed up successive penalties just short of the hour but instead of pressing home their obvious advantage Morgan Parra went for the posts and missed.
This was a huge let-off for Leinster and one Clermont almost lived to regret as, despite being in absolute control, they were eventually out-scored by two tries to one, which is a credit to the fighting qualities of the Leinster players.
You do not win three Heineken Cups without possessing an able band of talented and dedicated players and while they were visibly fighting a losing battle they did so with total honesty and endeavour and their reward was that precious losing point.
Whether it will be of significance when competition is resumed in the new year remains to be seen but one is left with the inevitable conclusion that if it is then they must have their injured and absent players back in action.
Never have Leinster missed the huge talent of Brian O'Driscoll more than in this contest. Quite apart from his powerful defensive work and deadly finishing, the absence of his routine contribution at ruck time cost Leinster dearly. Rob Kearney was also desperately missed. Ian Madigan remains a fine player but the truth is he is an out-half being pressed into service out of position. The return of the injured warriors cannot happen soon enough for Leinster.
That they retain an interest in the competition, however slim, is the consolation Leinster must take from this game – they never stopped working, they never stopped playing and they never stopped looking for an opportunity to strike. Their late try underlined that, even if it was obvious from an early stage that any hopes of scoring the type of win that would close the gap in the pool table with Clermont were fanciful in the extreme.
The Leinster defence has never had to put in a harder shift than Saturday's exercise.
Fofana, the villain for Clermont last season when he ruined their hopes of beating the champions by dropping the ball, was constantly asking questions of the Leinster defenders while Sitiveni Sivivatu and Napolioni Nalaga terrorised the home side down either flank.
There can be no denying Clermont should have won by a greater margin. They outclassed the champions like very few teams have been able to do and will, even allowing for the concession of that late try, be amongst the favourites to win the competition outright. Leinster are on life-support after this loss but they are not done just yet ... miracles do happen.
Leinster – I Madigan; F McFadden, G D'Arcy, A Goodman, I Nacewa (D Kearney 59); J Sexton, E Reddan (I Boss 66); H van der Merwe (C Healy 47), R Strauss (S Cronin 12), M Ross (M Bent 62); L Cullen (capt), D Browne (D Toner h-t); S O'Brien (yc 51-61), J Heaslip, S Jennings (J Murphy 69).
Clermont – L Byrne (R King 37); S Sivivatu, A Rougerie, W Fofana, N Nalaga; B James, M Parra; R Chaume (V Debaty 66), B Kayser (T Paulo 70), D Zirakashvili (D Kotze 68), N Hines (L Jacquet 75), J Cudmore, J Bonnaire, D Chouly, J Bardy (yc 76).
Ref – W Barnes (England)