Well, it's quite simple. Sevii Islands were FRLG's aftergame, it's own unique thing, but you're okay with putting them in ORAS' aftergame. So by the same logic, is there any problem with putting DPPt's aftergame of Battle, Survival and Resort Areas in BW's aftergame?
Birth Island and Navel Rock are in FRLG and Emerald, right next to a couple of the Sevii Islands.


So as you can see, they're quite close to the Sevii Islands. The Sevii Islands being included isn't actually that farfetched, all things considered.