We don‘t have any evidence. I‘m just basing my opinion on facts from previous expansions and PTS cycles. June is de facto the earliest month they can release 9.1. Everything later than that has yet to be seen, but the June date gets more and more unlikely the longer 9.1 isn’t on the PTR.
We have this rumor that an employee said June is optimistic and August likely, but never was September mentioned. That’s why I stick with my opinion based on the numbers we have: June earliest months although unlikely if TBC launches in June. July most likely month, August pretty much the latest possible option. Your September guess is based on nothing factual at this point.