I don’t think that’s quite correct. Works themselves are reasonably well defined. The work type field, on the other hand, is not.
So far it’s been used mainly for musical forms, but no definition was formalized (that I’m aware of).
As I understand it, nobody is saying that “instrumental” isn’t a valid descriptor for a work, but some people are saying that it doesn’t belong in a list of musical forms (which is the current de facto use of work type).
addendum:
There are two things you can do to mark a work as “instrumental” in the meantime.
- (as @Torc, @outsidecontext and others have suggested) use tags, which are meant for all kinds of description.
- (as @jesus2099 suggested) use annotations, which are meant to hold data that doesn’t fit anywhere else