Ukraine claims it would be a 2-3 week training process

I don't know shit about military aviation but type ratings (aircraft specific ratings to fly specific, more complicated to operate planes) take a lot longer than that to acquire unless they're absolutely rushing it, which is probably safe to assume. Plus, using the gun is a whole other beast in addition to learning a new plane. That thing puts out so much power it significantly slows the plane down when you fire it.