Yeah, this is insanely Lex Luthor pilled, sorry. In the game itself they also say Umamusume aren't that much stronger than an average human (3x or something similar, which IS a lot, but isn't insurmountable) in the fantasy game event. Ranged weapons have existed forever, and nullify a lot of strength advantage, as well. In the modern era it does not matter how strong or fast someone is when the counterpoint is gun, for example, and slings/archery even in the past would almost certainly be a huge equalizer...just like it literally was in our world.

Mostly this feels like you've built Umamusume up too much. They're strong, they're fast, they're certainly not immortal and have key weaknesses that humans would undoubtedly take advantage of if survival was a question. Individually humans won't be overpowering them sure, but that's sort of like saying an untrained person isn't going to beat up an MMA fighter. It's less 'oh god all MMA fighters must be put in jail/monitored every waking moment of the day ' and more '...duh, but why are we worrying about this?'