The other thing that also kinda supports that theory of that scene being earlier in the movie besides the long hair and mjolnir is that Loki apparently is having a hard time keeping some deals that Odin had in place when he was ruler of Asgard and Loki is losing his grip on the Nine Realms... so probably Thor comes to Asgard and after learning whats going on and Loki's schemes he needs to head to Muspelheim to fix things up. But then again why would he be chained up? Is it some trickery from Loki in order to get Thor down there and do what he gotta do...
It's a bit of a mystery at this point. I guess Thor could go down there and trick Surtur somehow to later on get screwed by Hela which in turn brings back Surtur to the fold. It's a lot of stuff going on and it's a massive jigsaw puzzle. But soon enough we are going to be able to pin point a lot of stuff.