@noxypaws their terms https://www.grammarly.com/terms#user-content at least seem to leave it open.
"You grant us a license to your User Content for the limited purposes of: ...
"Developing new products or features (for example, creating our tone detector)"
Together with the fact that they bill themselves as an AI writing assistant service, it's not hard to put 2 and 2 together and no one should be surprised if this was going on awhile.
@ljwrites @susankayequinn I wouldn't put it past them, or anybody, at the moment. but it should be much bigger news if there was any sort of revelation recently.