In the big battle scene, there were so many extras that they didn't have enough budget to shod them all correctly. If you look closely you can see that many of the extras are wearing sneakers.
Sneakers were invented in Rome, I researched this, besides, the extras would be asked to bring sandals, I've been an extra, and was asked to bring clothes, it is not uncommon, and you are wrong. They also could have gone barefoot, as extras are treated poorly.
im sure the director isn’t as stupid to let people wear sneakers. i mean, if you were offered to star in a film with a prerequisite of brining sandals, what would you do...