what else could it possibly be if not that?
The advancements in shoes have made a measurable impact, but there are lots of optimizations being worked on.
If you're bloody quick and born in Birmingham (either of them) in 1900 you can probably find out about and get yourself a chance to attempt that world record, but if you're born in Kapsabet (in Kenya) in 1900 good luck, even in Nairobi I wouldn't bet on it.
But yeah at this point, “it’s the shoes, stupid” should defo be the main part of the conversation.