the wiki says that in the B2 universe, the roles are reversed (dave and rose are roxy and dirk's parents, poppop and grandma english are jake and jane's parents). it's under "ectobiology", click "post-scratch" above the diagram at the bottom

They are their “parents” socially since they arrived earlier, but not genetically.  Like how the Ancestors are technically the spawn of Karkat’s group.