A transformer, regardless of what it is trained to do, is just a pass thru architecture consisting of a fixed number of layers, no feedback paths, and no memory from one input to the next. Most of it's limitations (wrt AGI) stem from the architecture. How you train it, and on what, can't change that.

Narrow skills like playing Chess (DeepBlue), Go, or math proofs are impressive in some sense, but not the same as generality and/or intelligence which are the hallmarks of AGI. Note that AlphaProof, as the same suggests, has more in common with AlphaGo and AlphaFold than a plain transformer. It's a hybrid neuro-symbolic approach where the real power is coming from the search/verification component. Sure, RL can do some impressive things when the right problem presents itself, but it's not a silver bullet to all machine learning problems, and few outside of David Silver think it's going to be the/a way to achieve AGI.