If we're talking about Demon Gods, i'm fine with Clan Calatin being Island level. Individual Demon Pillars though? No way. Mashu soloe'd Flauros at the beggining of Salomon, and in that same chapter, a bunch of Servants were absolutely massacring multiple Demon Pillars at the same time, over and over and over again without using their NPs. IIRC, the only NPs used were Drake's, Blackbeard's and Altera's. And regardless, the vast majority of them don't have Mountain level NPs to begin with. In the CCC event, Gawain and Meltlilith (and possibly Tamacat, i don't remember if she was with them) defeated a weakened Demon Pillar without using any NPs. Really, the only basis for Island level Demon Gods is a composite of 28 of them survivng a combination attack from Edison and Tesla, that was twenty eight, not one. That feat shouldn't scale to anyone, except maybe King Hassan, since he killed eleven of Tiamat's elite Lahmus that were explicitly stated to be stronger than a Demon God, but even that feels kinda like reaching tbh.
I haven't started Septem on NA yet, but from what i remember of the Japanese version, Altera was buffed by the Grail, wasn't she?