EDIT: Clarified that the logic outlined herein is best explained as parallel modulation between the MELODIC minor (not natural minor) and major key (see explanation below).
Well there does not seem to be much agreement on what key the song is in and I have seen no completely convincing arguments that put this progression solidly in any one particular key.
I have listened to the sound file and played it myself using the Cm - Am progression and the one played in the recording (Dm - Bm).
What I can say is that is a very effective piece of song composition and succeeds beautifully at conveying a very unique and distinct feeling. To me it defiantly feels more minor than major and seems to create an ambivalent feeling of uncertainty. Or a feeling like the waves coming to shore and then rolling back to sea. Very melancholy as well.
I am going to say that (aside from the simple fact that the Cm chord is not often used in the key of Cmaj) since it really feels/sounds very minor, it's difficult to reconcile it being only in C major (or any major key given 75% minor chords). So I would say that it is an arrangement that to a very good effect uses a subtle but constant back and forth parallel modulation between C melodic minor and C major.
C melodic minor contains the Cm the Gmaj and the Dm but would use an A half diminished chord (A C Eb) instead of an A minor. If we used AdimA half diminished instead of Amin in the progression - it would fit squarely in the key of "C melodic minor" (not C natural minor or C harmonic minor).
And since the G major chord is in both the melodic minor and major parallel keys, it's difficult to tell exactly where the modulation takes place and that to me is part of what makes this progression so powerful. It's like watching waves come to shore and back - you can't really tell exactly when the wave stops coming in and starts to go back out (what point they actually reverse direction).
The ongoing back and forth parallel modulation creates a yin - yang, ebb and flow effect. Perhaps unconventional, but again very effective at conveying a certain swaying movement to the music and certain emotional feeling.
So rather than try to put it in a box and say it's definitely in this key or that, I think we should just agree that it can be described as a very effective use of parallel modulation between the melodic minor and major key of whatever tonic chord we open the song with.
I see this as a great example of a composer willing to be inventive and think outside the box. Sometimes this type of unconventional device is beyond classification. Other than to just say it's brilliant!