How do you "acqui-hire" a bunch of people from a company without buying the company itself? Isn't that just, you know, hiring?
Maybe the employees signed a non-compete agreement, and what Apple is buying from Color is a waiver on those agreements? Non-competes don't have a great record of standing up in court, but OTOH $2 million split 20 ways isn't that much money per person either; might be considered cheaper than the risk of a legal challenge. I dunno.
The reporting on this story could definitely do with some additional detail, but given how rapidly the story's been changing we may just have to wait an hour or two for that detail to be uncovered :D
The employees, of course, are free to walk away from those unvested options and retention bonus, should they chose to do so.