If "Apple Music library" is the same as "all music sold in the iTunes Music Store", then why not say that? Why not be crystal clear on the issue? Why say "purchased in the iTunes Store" and "ever-expanding Apple Music library" are different things if they are referring to the same experience.
I don't want to suggest there is some conspiracy or anything, but it's not very Apple like to leave any possible confusion.
I would bet money that Apple does not have contracts yet to allow ALL the music sold in the iTunes Store to be streams. Hence the wording. My guess is that Apple Music Library does not equal iTunes Store Library.