I mean… Okay Silvermoon has had Alliance sentiments since ever with that little rally in the square and the magisters having to mind control them, and I kind of take Umbric and his followers as just being with Blood Elf society, the Silvermoon scholarly types minding their business doing their research.
I don’t think its THAT far fetched to assume they stuck with their kin as Sin’dorei researching the Void not really caring one way or another who Quelthalas sided with politically, that’s why when Umbric comments “we had no love for the horde anyways” it kind of makes sense, I’ve never heard of Umbric before the Void Elf introduction he wasn’t some right hand of Sylvanas or Lorthemar he was just in Silvermoon with some Silvermoon citizens living his life, so when he and his allies did decide to become politically affiliated it was kind of like “We remember the alliance we like them” type attitude