Vo2 max, efficiency and lactate threshold
Back when sports science began in the 1890s with guys like Philippe Tissié, most of the thinking involved finding ways of overcoming the pain barrier. This involved doping, a part of sports science but not the totality of it. Stimulants allowed the body to be pushed further, other products dulled the pain.
In the 1970s or so, sports science began to look at oxygen delivery and uptake., VO2 max and lactate threshold (the latter can be calculated as a function of the former). That became the dominant thinking through to the noughties. Now I mentioned Moser's Hour earlier. At the time, that Hour was fully legal. It was only in later years that the way it worked around the oxygen limitations - blood transfusions - were made illegal (and it was only years later that the technology of the bike was deemed illegal). Oxygen uptake wasn't the only part of Moser's Hour, there was some counter-intuitive thinking with regards to the bike (it was heavier than then traditional thinking said it should be). Like this sub-2 run, all areas were considered, but we tend to focus on one as the key element.
In the last decade or so, the science seems to have moved on from oxygen to fuelling and we're having to get our heads around things like L-Carnitine and Ketones. Get past the laser pacing (similar to a technique used by by the American rider Willie Hamilton in his 1898 Hour record), the allegedly counter-intuitive aerodynamic formation, the pacing strategy, and the shoes, and the narrative for this sub-2 run revolves around fuelling. In the last attempt, they say they got the fuelling wrong at the end, hence his ragged finish then. This time, they say they got it right.
Point here is that setting the physiological limits of man as oxygen-based seems to miss where the thinking is actually at today.
I’m convinced he could have gone even quicker than he did yesterday by a significant margin
I would have thought that that was a given? The strategy was to break the sub-2 barrier. This is the way with records. from the Hour to the pole vault and all in between: generally speaking, you aim to break the record, only rarely do you aim to put in on the shelf and thoroughly smash it.
So if the the guy had a pacing strategy to get sub-2 without tripping over his lactate threshold - and a fuelling strategy that enabled that pace to be held to the end - then the flags you're seeing at the end, him not being on his knees and crawling across the line like we imagine a true hero should, aren't really flags.
But, then, we're back to where the physiological limits are: if you insist the physiological limits make sub-2 physically impossible then you're not going to accept that the new record is already ripe for breaking.