I see what you are saying. I think it depends on the understanding of their rules. And whether they have the agency to shutdown themselves (which is my understanding), or whether the whole omniscient system is responsible for the shutdown (which I believe is your understanding).
it is open to interpretation. the idea was that shutting down keeps the harmony in the society (because only once they know their own color they have some sort of incentive for discrimination against other colors).
and yes, there is a question on trust and belief versus knowledge. but assuming Vulcans are trustworthy, I think the synths conclusions can be seen as knowledge (i.e. not just belief).