Most of those world champions didn't have the internet where every blitz game can be saved and quickly referenced against a massive database and a few positions checked with an engine.
It doesn't replace deep study, but it's instant feedback.
You would never be able to get this from blitz 50 years ago.
Every single world champion except Botvinnik not only played blitz, they excelled at blitz. Every elite player today plays blitz. Almost every young GM today plays tons of blitz online.
Blitz alone won't make a player a GM. But, blitz is an extremely useful tool when used in moderation. The problem is that it's so addictive that many people never get around to studying positions deeply.