Even if he was holding a trumpet, the guy is still Bix. He is perfectly recognizable. (I make my money drawing people.) Bix played in the trumpet section of the Whiteman Orchestra which, according to the orchestra's programme, means he would've been playing trumpets, cornets, flugelhorns. If you can play one of those instruments, you are perfectly capable of playing all three, (though not necessarily at the same time).
The cornets Bix was most associated with, were all long models, (which are longer than the traditional cornet) they were built to project more like a trumpet does. My 1917 Conn Victor, is only about a quarter of an inch shorter, than my husband's Bobby Shew Yamaha trumpet. Such a miniscule difference, in length, would not be detectable in these photos.