That is not guaranteed to work. In another thread it was pointed out that this is merely a CPython implementation detail, NOT a language feature.
I'm curious where this is written down. Can you point to the relevant part of the language spec or pronouncement or whatever it was?
I'm going to be charitable here and suggest that Chris is referring to something implicit. The spec clearly says that identifiers must be valid keys because otherwise normal functions wouldn't work. But that's it. So in Chris' view you're probably asking him to prove a negative. The burden of proof would lie on those who say otherwise to show how the spec allows arbitrary strings. But no one has made such a claim so it's a bit moot.