For most files that you actually save, yes. They will be saved in general storage. In this point you are right, and there are no problems in deleting these files.
However, if you try opening files attached to, for example, an email without saving them, then Word will implicitly save them in it's own data folder (the one mentioned in this thread, that has path like /data/user/xxxxxxxx....) and will use them. And no
other application has access to another application's data folder. And it appears that Word itself fails somehow to delete these files. How? I have no idea.