Black Templar worked with Eldar far before the Gathering Storm things. Sometimes, you just don't have the choice.



As for the Primaris, it is said in the Adeptus Custodes Codex that the Custodes just say to all Chapter who wanted to refuse the Primaris that doing so would be refusing an order of the Emperor himself, which would turn them for traitor.



Even Black Templar can't really argue against that...