This speaks to a failure of reading comprehension. You are told by Gweyr, going in, that you shouldn't keep fighting but withdraw after you feel you've done enough. If you keep on going to the point that the game runs out of letters that's entirely your fault for missing not only her dialogue but the
bolded prompt right before the battle triggers that tells you this.
Like I said, a total failure of reading comprehension on your part:
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You are explicitly told this is not a fight you can win and that you should
run away once you've done your bit. Having Kas show up to tell you that you aren't meant to win would break the fourth wall
and the narrative, since what's going on up in the Rift is Tollus doing his own thing. There is likewise no
reason for the game to 'just give you the win' here when it's made crystal-clear that you're not
meant to win, but to distract the cultists for a while.
The devs have been clear that they only milestone conduct that they want to encourage. People trying for some kind of record in a fight that explicitly isn't meant to be won isn't something they care to encourage.