If I'm understanding correctly you're asking if we need the "One Room" theme when Halloween can also be viewed as a theme in itself?
To answer that, yes I guess Halloween can be viewed as a theme to base your game around. You could also make a game that's spooky which fit's nicely into this game jam but has nothing to do with halloween which is the route most people seem to take. Because of that I decided it would be nice to have the "One Room" theme to help inspire people.
Either way the "One Room" theme is optional meaning that is is not necessary for your game to have any ties to it and the game also doesn't need to be specifically about Halloween.
I hope I correctly understood what you asked and gave you an answer If not or if you have any other questions feel free to ask!