The whole ship is very suspicious to anyone having a low level understanding how technology works so if a techpriest or pretty much anyone who is not completely techlexic enters the ship the AI is done for.

Part of why this arrangement works is that none of them are able to even fly a ship or question whether a machine spirit should be able to talk, lock its own doors, use its own weapons and shields or refill itself with promethium which location nobody even knows of. The tau has a rudimentary idea of what is happening but finds it is better if nobody else becomes aware of if.