Nah, they’ve way too much stock in Ironforge, and Moira, and her son, etc.
Dark Irons being a part of the Alliance has been a thing ever since Moira has been preggers with Thaurissan’s son, since Vanilla. There were Dark Irons that didn’t follow Ragnaros, and wanted out, like the neutral groups.
The seeds have been sown already, and the Dwarven tribes, all 3 of 'em, used to be allies way back in the day. They fought over Ironforge and the Bronzebeards won, kicking the other two tribes out to forge their own lands.
So, we’re just going back to what already was.
If Dark Irons ended up on Horde, it would’ve been really awkward for Moira, as a Bronzebeard Alliance Dwarf, herself.