My understanding is that the initiation exclusivity pertains to NPC-to-NPC group and NPHC-to-NPHC. So an initiate of a MCGLO could be initiated into an NPHC or an NPC. Or, technically, both.
I'm pretty sure that I've seen posts from NPHC members saying that initiation into an NPC sorority is a bar to initiation in at least some, and maybe all, NPHC sororities.