I don't think the song is polytonal - but the perceived dissonance is there. Lorn in the song uses some noticeably detuned synthesisers - not so out-of-tune as to change how we would classify the note, but out-of-tune enough to be noticeably 'wrong'.
What makes the effect all the more potent is the fact that (to my ears at least) the tuning of the synths wavers as well - so the synths fluctuate between being more and less out-of-tune. The whole thing creates an unsettling instrumental, over which the vocals sit. (Interestingly, because the vocals don't waver in pitching as much as the synthesisers do, they actually contribute to the feeling of 'detunedness' even more.)
Combine this with complex chords and progressions (including a lot of suspensions), and the whole thing combines to create this perceived dissonance. So it's not polytonality that's at work, but a combination of intricate harmony and combinations of varying tunings.
Hope that helps!