It seemed so, at least, in a Byzantium campaign, while I was looking at Crete, which was involved in an Italian Civil War. One turn it had 10 handgunners or so, and three turns later it had 13 (or so it seemed to me)

So was I mistaken in my count, was it a leftover queued before the civil war, or does the rebel AI build units if the buildings are avaiable?

If the latter, wouldn´t it be interesting to mod the starting buildings of "rebel kingdoms" (Navarre, Portugal, Khazar...) so that having neighbouring rebels is actually a threat of sorts?
Also, what about making the rebel AI able to build the infrastructure for basic units (Militia, archers) if possible?