It is very unparsimonious to assume that specific human signaling behaviors are inborn, given that there are such an incredible number of such behaviors in use.

I agree that the vast majority of specific human behaviors, signaling or otherwise are learned, not in-born, as an Occam prior would suggest. That does not, however, mean that all signaling behaviors are learned. Many animals have instinctual mating rituals, and it would be quite surprising if the evolutionary pressures that enable these to develop in other species were entirely absent in humans.

Much easier to assume that signal detection and self-reflection add up to standard reinforcement, as signal-detection and self-reflection are independently useful, while standalone signaling behaviors are not.

I would expect signaling to show up both in reinforced behaviors and in the rewards themselves (the feeling of having signaled a given trait could feel rewarding). Again, most are probably behaviors that have been rewarded or learned memetically, but given the large and diverse signaling behaviors, the more complex explanation probably applies to some (but not most) of them.