Few would claim molecular assemblers are physically impossible. Drexler made claims about what was possible under physical law. They are pretty conservative.
I don’t think he ever pretended to having technology he didn’t have.
I do think Drexler committed the grave sin of writing a book for popular consumption. He should have published Nanosystems first and waited decades before writing Engines of Creation.
Now a days, Drexler seems to make a living opinionating on "hard AI" -similar level of vaporware. I mean, whiggism is the default national religion of the US and the West in general, and I guess we need our preachers, but his career is preposterous, and the ideas he peddles are beyond snake oil.
The point of Nanosystems was to explore what technologies are feasible given our current understanding of physics at that scale. It turns out there is a lot of latent potential there.