As far as I can tell, enableAnnouncements is used as a global setting and therefore ignored on the item level. Items use a different field, creatively named announce, which should toggle announcements for the given item.

If this is true, disabling announcements for a custom (imported) item shouldn't work.

This should be very easy to fix, though some double-checking is needed to make sure I got it right. Also, the item pack (listed on WowAce as an import string) may have to be updated, if re-importing the items doesn't automatically resolve the problem.