The odd cutscene with Y'shtola and Runar in 5.3

[Justuas]

Interdimensional travel has been said to be impossible but obviously it's not, so figuring out how to do it is a challenge she's willing to tackle when there's spare time.

I've finished 5.5 MSQ, but when I go to The Empty I see Thancred and Urianger there. If they can hop between worlds then why not Y'shtola too.