So from that, is it possible that it didn't happen?
Well, the hypothesis is that the details in Matthew 28 about the guards and the great commission are later additions.
But the disciples going to Galilee (Mt 28:16) follows naturally from Jesus' words, "after I have risen, I will go before you to Galilee" in chapter 26. So it's not clear cut.
If Mark and Luke were following Matthew up to the angel at the tomb, the fact that they don't include the appearance in Galilee suggests it wasn't in their copies of Matthew. So Luke got his details of the appearances from another source and Mark might have done the same.
If the resurrection did happen, more than one person would be available as a witness to appearances, so for Matthew not to include one doesn't mean it didn't happen, as Luke and John have independent accounts.