I just went and tested it myself. I upped a Yak-9T and had a player that had no idea about the situation up an La-7. I tried several times to send him a join request (using .join and the clipboard both) and he never got a single request. Then we switched it up (he sending me the requests) with the same results. You CANNOT send join requests while in an aircraft or vehicle. Duh, that's solved.
The only question left is: "Who's lying?"
Moray's account of the story says he flying a Yak-9T at the time, so he couldn't possibly send the request.
LYNX, on the other hand, provided a screenshot showing himself receiving multiple join requests from Moray, then Moray denying the accusation.
Which is the truth? You decide