I could be wrong, but I think it has to do with the date he was signed (he was signed on Nov. 7, 2016). I'm just not exactly sure when that date is (i.e. before the season)
edit: that doesn't make much sense otherwise a guy like McAvoy wouldn't be allowed to play right now. I'm assuming he'd be eligible but I'd expect the Otters to make a long run to the Memorial Cup