I'm pretty sure it's not a harmonium. Harmoniums have more of a resonating sound, almost as if more than one note is being played at once. They are neat. They usually have a bit of a deeper sound than what you hear on More Than This too, from what I've heard of them.
I'm not an expert on any instruments, but from what I've seen of the harmonium, more than one note
does play at the same time.
For an example that may not be correct: Say you play middle C. The C an octave higher would be played automatically as well. If you want, I can study into that a bit more...