It'd sure be a big friggin' help if the guideline gave a clue as to why the hell that is. Anybody around when that decision was made? Was it because it was premiered before the movie opened? Is it a derivative of the score that doesn't actually appear in the movie? Or is the idea that only the top-level full score be set to "Soundtrack", but portions of it should not.
Judging from the other "parts" of the ESB work, it looks like that was the intent. None of the other parts are marked as a "Soundtrack" work type, but they do appear on the ESB release, which is marked as a Soundtrack.
So apparently the Work and Release definitions for the Soundtrack type aren't consistent. I don't interpret the definition as meaning a compilation of music from soundtracks isn't actually a soundtrack, or that it has to be the original recording. It's still music that was originally composed for film (or TV, etc.). Besides, we use the Soundtrack type for Theatre releases too, regardless of when it was recorded.
(Or else you could interpret it the other way and say soundtrack means the same thing for work as it does for release, and the definition that "a soundtrack is the musical score to a movie..." really does mean not to use Soundtrack unless it's the entire score to a single film.)