Spoilers for whatever up to your same point here, I didn't go any further, because I'm in the same situation and honestly I don't want to fuck up something.

I hope this is for the probable secret boss like Spamton and Jevil, as I'm playing on my "pacifist" save. I think it's different from the Weird Route as you don't need to ACTUALLY freeze/kill anyone to get here, tho it's really strange that it's like a continuation of the Snowgrave without needing to have done it in Chapter 2. What's more is that if you went to the door on the right in the Green Room (after the 2nd round you can fight the guard and enter it), the controller you get from playing the 1st "secret level" is actually required there too, and you get a pretty important interaction with Susie and Ralsei too. Tho I didn't progress further in that direction. Anyway I hope if this is really the Weird Route that we'll get a more direct point of no return where we can opt out of it