This effect seems slightly redundant, since as far as I know, only Hybrids can evolve from Tamer cards. Though I suppose this opens up the possibility of non-Hybrids utilizing the same mechanic; first to come to mind being Hudiemon.
Well if it just said tamer- then the easy way to get around it is to just get rid of that tamer evolution material. Play card that can remove the bottom evolution material and you opened that Beowolfmon up to attack. But yeah they could of just mentioned a hybrid material since IF There is a tamer in the stack- there's also a hybrid in the stack. They could of definitely been taking the extra step to future proof the effect.
What if we got BioCoatl, Bio Thunderbir, and Bio Stegomon in a couple sets (or even somewhere in this set) they would clearly be lv 4 digimon who could easily evolve from a tamer without being a hybrid. I could probably even think up some sort of way to integrate Tamer's Biomerge/Matrix evolution set up though that'd be clunky and probably be messier than needed.