He may be breaking rercords, but what in his history are you comparing to? He isn't ever going to break the 10km for example. So you do not know if he is physiologically getting better with age, because you do not have any phyiological data. Without that you are just speculating
He is (they are) probably just bringing the marahon more in line with the physiological limits compared to other distances, as it has been long speculated to be out of line. The things that are enabling that are ,
1) most importantly running economy, and if they were to give you data on VO2/kg/km (not VO2max) he could be closer to Tadesse like numbers now and he has almost a faultless style.
2) Add the shoes, which do not 'enhance' (i.e. no external energy), but lose less energy, then that economy is driven even further down. So both of these mean faster with the same (or even slightly declining) engine
3) Add proper pace judgement, pacers (as evidenced by Vienna where they rotated) then you limit the fade.
There's probably a few other things of lesser value, nutrition, monitoring of physiology, etc etc
Lastly, he is 'out there' available to be tested, they have his blood passport and have access to the O2 consumption.