I can’t answer for Psionix, because… I don’t work for Psionix. But having worked on other projects, I can tell you that a fully-authoritative server (that’s the word you’re looking for, btw) is not the end-all-be-all of anticheat. Every game has different levels of mechanical complexity, logical complexity, and a myriad of other variables that factor into what type of architecture is used in online games, and that a fully-authoritative server not only isn’t feasible for all projects, but also isn’t a silver bullet against cheaters.
Now I’m curious what would still be possible to cheat in such an environment. Unless you are merely suggesting that the server itself could be hacked; I do understand that would end up being the target in this case.
So forgive me for not knowing the term, but there’s a type of attack that waits to send commands until after packets have been received whose data provides an advantage. For instance, a bot could simply wait until it receives the position of your opponent, calculate how far to turn the player to aim, then tell the server “I’ve moved the mouse in this vector”
A bit like playing rock-paper-scissors, but waiting until you opponent shows what they’ve chosen before making your own decision.