I believe what he’s referring to in that segment is that you cannot level a basic, full grown pet into a greater version. If you pull a grown, basic shaleback from your animal pen and level it up, it will never become a greater shaleback. With proper feeding and some leveling luck in the perks, your basic version fully leveled might turn out preferable to your greater version fully leveled.
You do, however, have a chance to get the greater version from the babies that operates just as it did before (I’ve gotten the greater versions of tigers, shaleback, and hyena so far).
For example, I put two baby shalebacks into the pen and fed them desert berries. I got 1 basic and 1 greater. The greater starts off with much higher HP (2,257 vs. 296) and generally all around better stats. Through the leveling process, there is a chance that my basic shaleback will be better than my greater, but my basic shaleback will never evolve into the greater form.