Firstly, he said he has never worked with an effective architect: you have changed his meaning completely to imply he doesn't think architecture matters.

Secondly, you can't know he hasn't worked on mature systems (he didn't say). Most people learn by working with a variety of systems that are at different stages (greenfield to twilight), and more importantly with different quality (poor versus good architecture/systems/teams).

A long period working on one good system may help a few people learn, but from what I have seen, it often doesn't (I have worked with crap programmers on mature systems).

Hopefully GP answers for themselves.