But according to the README, the new language is syntactically incompatible with the old VimScript. So you can’t reuse those extensions, they have to be rewritten.
Yeah, I didn't notice that. If that is the case, then perhaps there is no purpose in creating a new language. Perhaps the only benefit to creating the new language could be that it might be a little easier to mechanically convert from the existing VimScript to the new style VimScript they are thinking of.