Trying to conceal intelligence would have entailed pretending to be dumb at virtually all times, which implies giving up most of the benefits of being intelligent.

There would still be benefits if your model is at all accurate and there are 'secret rounds' in ordinary human life. Just pretend to be stupid in public and then be smart in private rounds. To frustrate this, one would need to assume that the additional smartness costs too much. (It is so expensive that it outweighs the gains, or the gains are minimal so any cost outweighs them.)

It seems reasonable to me that there are private rounds in real life and that smartness is a net win.