If you do assign an interface, no need to give it an IP, it is automatically given the IP of the tunnel as defined in WireGuard.
Since you did not post your exact config it is impossible to verify your claims. (No matter, my setup works now and I don't really care.) Either way, in my setup it only started to work when I followed @rman50's advice.
why would you assume that while trying to help you I would just make stuff up?
I bet after adding your interface without defining IPs that you didn't stop and start WG, right? Without doing that, yes IPs aren't assigned. But once you do stop and start WG, the IPs are there.