How is this unassisted?

I've seen this happen a couple of times.  In the ones I've seen the goalie catches shot, but the save animation immediately pulls the glove and puck into the net which is being treated as a goal.   My best guess is that the save counts as a change of possession so only the last player to touch the puck on the scoring team gets any credit for the goal, there's technically no assists.

I can't be 100% positive that's what 's happening in your clip, but it certainly looks like it.