My experience is that DefinitelyTyped definitions are actually not very accurate in practice. This may be a consequence of language restrictions or just typical

My experience is that DefinitelyTyped definitions are actually not very accurate in practice. This may be a consequence of language restrictions or just typical (and understandable) programmer laziness ("good enough!") -- or, as you say, the lack of type unions. It's better than nothing, but not really good enough. But then I have been spoiled by Ocaml and Haskell, so... (There's also the whole external/internal module confusion in TS, but that's not related to the general theme of gradual typing per se, so it's not really relevant to non-TS contexts.)