My boss at my last job had done the opposite. Coming from a tradtional compsci C/C++ background (many moons ago), he learnt Angular 2 for a frontend project that required it. Both his C++ and frontend code were among the best at our company.
Saying there aren't good fullstack engineers is like saying multi-instrumentalists aren't good musicians. Sure, the very very best violinists probably don't play other instruments at a high level. But the average multi-instrumentalist (who plays the violin) is a lot better than the average violinist.
What actually is your point? Reading between the lines, it seems to be:
"Anyone who calls themselves a full-stack developer is garbage - it's just a matter of what side your garbage is on."