This was a talking point back when he was a junior and he even had a more extreme position. The reason is that he is very nimble, which allows him to stay in position while putting out bigger numbers compared to riders who are less nimble, who won't be able to maintain that position all the time and won't be able to output those numbers in said position. This might be one of the advantages of having been a football player until the age of 17.
So in the windtunnel they figured out over the years that his junior TT position, while extreme, was not the most efficient. They have since also put shorter cranks on his TT bike, which allows for a better leg angle. You should read up on that, for me it kind of feels like it would be counterproductive but apparently a shorter crank, while not being able to push the same power on it from a lever-effect point of view, has this off-set by gearing ratio and the speed you gain by taking less time to make one spin since the circle of the spin is smaller due to the shorter cranks.
Junior TT position (look at the aerobar, flat on the handle bar).
Current TT position (see the difference in how high the aerobar is mounted and its angle)