The MEV bots had a protocol that they were expected to follow. These two guys were running a secret cluster of their own bots that strongly violated the expected protocol.
You could alternatively claim that the guys defined their own protocol which addressed market inefficiency (which MEV is). Imo it's insane to claim that a trading technique you invented should have zero risk, and any losses you take are an indication of theft.
Consider this simple case: if you visit me, and I show you a paper with keys to 0.1 BTC. You secretly take a picture of it while I step into the bathroom for a minute. Later you go home and you drain the wallet. In this scenario, did you really do a crime? You only violated protocol. It is not illegal to try random BTC keys. But I think, yes, you did.