This seems like a strange idea given the supermassive black holes we’re finding in the early universe. That’s significant because early stars had low metallicit

This seems like a strange idea given the supermassive black holes we’re finding in the early universe. That’s significant because early stars had low metallicity which means that metals were very rare, and were so until recently (gen 3 stars). If civilizations were turning themselves into black holes, they had to do so without much of what we consider technology. Certainly nothing like what goes into an EV, for instance.