He was exempt as he was under contract at the time of Gatlands Law, and he did an extension. But I think he would need a new contract for next year to stay at Wasps, and the WRFU may have said, 'Look, we want you. We are allowed to select and play you in the 6N, but we will only do so if you move for next season, then we can also give you RWC selection.' YY would have said, 'You pick and play me in the 6N, and then the RWC, and I will move.'
Totally and utterly conjecture, but much the same happened with Shields, yes? And, I suspect you can find examples of Welsh players recently with much the same scenario.