I think they mean backwards-compatible syntax-wise, rather than actually allowing this feature to be used on existing code. If I’m understanding correctly they would prefer for the Python grammar to stay the same (hence the comment about updating parsers and IDEs).

But I don’t think I really agree, the extensible annotation syntaxes they mention always feel clunky and awkward to me. For a first-party language feature (especially used as often as this will be), I think dedicated syntax seems right.