Not really sure. Besides Rasanen and Bouthillier, everyone else on the roster had AHL or NHL deals. Well technically Sestito is still on a PTO but he's been here for like 4 months now... I doubt it's considered a tryout anymore.
The Marlies have like 30 players on their roster right now. I am surprised that many guys are on their playoff roster TBH... Especially since McGregor is better than many of them.
EDIT: For reference, Trey Bradley is also not on the roster, and he was signed to an AHL deal. Brazeau is not on the roster despite getting a two-year AHL deal, but that is somewhat understandable. Ryan Moore IS on the roster despite not playing a game for the Marlies this year. Who really knows what is up, but I wouldn't look into it that much. I think McGregor has a pretty good shot.