I have to assume it's correct, since I can't find anywhere where Nachtmystium is described as belonging to any sub-genre of progressive rock music, whether through press material, Wikipedia, etc.. I've yet to listen to the band, but if they don't belong to the genre, I guess you can justify the use of quotation marks.
Hell, I can do that, too, PunkNews. Make me an editor and I'll be pleased to report on the "Psychedelic" band The Gaslight Anthem and legendary "Dixieland" punk group Bad Religion.
I'd say that calling them "progressive" black metal is pretty much right on the money.
I'm a big fan of the last record and every single song pushes black metal into a different corner of the musical spectrum. There's some full on Entombed-sounding almost punk/roll stuff, but also some Joy Division covering Burzum sounding stuff. It's a great record.
If I remember correctly, years ago, a label, which was a subsidiary of a nazi label, re-released some demo material of theirs. (You may, or may not, be aware nazis dig em some black metal.)
Why is "progressive" in quotes? Not groundbreaking enough for you, eh?