That third period is going to give nightmares to every GM thinking about trading away one of their stars, and has a good chance of becoming an evergreen anecdotal example of a player sinking their old team. Oh, and it's also apparently the first ever game 7 third period hattrick in NHL history.
Rantanen had been an extremely visible force on the ice before he actually hit the back of the net, and that's how you end up with "fluke" goals
That 2nd goal is nothing compared to MacKinnons game winner in the last game after all.
Edit: here's another weird data point:
Rantanen scored a hattrick in both the first and last game of Colorados season
(The first game was also a loss, against Vegas.)