Master x Master is not GW2 canon and even if it was we never got an animated episode showing his trip to the Mists, only the GW2 cut scene explaining what happened with Balthazar. There's all manner of things that could have happened between him jumping in and escaping with Sohothin.

There is no such thing as "not canon" in a setting with a multiverse.

There is if it's never mentioned or alluded to within the original franchise. My own personal story of how Canach actually saved the world by 80s disco dancing isn't suddenly canon because it happened in an alternate universe...

Congratulations. There has ever been a version of Tyria in which that happened. Making up the tale only made you aware of it. It's Myst ages rules. You didn't come up with it. It was always there. Making up the tale only made you aware of it.