I used to think that maybe that gemara in Megilla doesn’t refer to the actual text, but to the explanations contained therein. As Tos. in Brachos 8b (top) says that the Targum often contains things which one wouldn’t have known just by reading the psukim. However, even this is problematic, because the rishonim generally don’t hesitate to argue on the Targum, which is strange if it actually was given at Sinai. I don’t really have a clarity on the issue.
However, all I was trying to say in my point was that this business of ????? ????? has its source in this gemara, not in ?? ????? ???? ?????? ???? ?????.
Wolf-
The gemara (Megilla ad loc) says it was forgotten and Onkeles restored it.