I'm only saying: Some skills are so radically different that you cannot transfer easily from one to the other. To give a counter example in the other direction: When i taught a former Lisp developer Perl, it took him 3 months to get to a good level, and 3 more to be on a very high level. And he's an extremely bright guy.
Then again, he had never done web apps before. Maybe the language matters a lot less if you're staying within the same problem domain.
If you asked me tomorrow to go and start doing system programming in a language I knew well, I'd probably be screwed. Ask me to work on a database engine, compiler or web app and I'd be okay.
I'm mainly focusing that ire on the typical enterprise stuff LOB stuff, where it truely is all the same and just because you'd been writing Foo v1 for three years doesn't mean you won't be able to write Bar v300
I feel confident that if I was asked to do C at one of these gigs I'd have been able to deliver something useful. Perhaps less if it was C++ because it's a bit swiss-army knife and there are a lot more "don'ts" to pick up.