The term "progressive" in a musical sense is pretty open to interpretation.
I'd say they're progressive in that each album has 'progressed' greatly from its predecessor. Each album is VERY different to the last. They also write fairly unique songs that have structure and 'vibe' that you don't get from other bands.
I'd say these points make them progressive, but if you look at progressive metal as something that is all about cramming as many odd time signatures, tempo changes, and guitar solos in each song as possible then no, they're not progressive.