True...but theory and practice aren't the same.
Drawings on a chalk board indicates the ponies have some sense of physics, it doesn't mean however they follow the same exact physics we follow here out of screen, nor how that said physics apply exactly to the carts in detail.
Not to mention there's never details about said physics since once the derby was mentioned no explanations comes out from that. For all accounts it could be like using magic environment or transforming combustion into energy.
I dare to say it was more of a setup gag taken too seriously. You can theorize but wasn't something to dwell into.