I have been a guest for over a year now, I guess the time is over due to become a member.
This is my debunking of this rumor:
Set number 75143 was originally listed as an A-Wing on Brickset when the summer wave was first listed there. It was also listed as a Rebels themed set. At the same time set number 75150 was listed as Darth Vader's TIE Advanced, also a Rebels themed set. Closer to the release date both sets were updated on Brickset to reflect their current status. 75143 was changed to an unknown Star Wars set and 75150 was changed to Vader's TIE Advanced vs. A-Wing.
I believe that TLG originally intend for 75150 to be released as 2 separate sets but combined them for some reason I will not speculate on. This is supported by the fact that the set contains 2 small instruction books, one separate book for each vehicle rather than a single larger book.