I know about Alcest..a guy on the Sound Opinions forum compared them to dredg so it got me curious. Their album was interesting for a short while, not a top record for me, but not the worst of 2007 either.
While I love Opeth, it is surely death metal. Given it is Prog Death, but it is much death as it is Prog (except their Damnation release). So I can see where the comment came from. But I agree it is not pure death metal like Nile, Behemoth, or Cannibal Corpse (which I personally have no interest in).
Indeed they can-- saw them a couple weeks back as a headliner and their set consisted of a pretty flawless start-to-finish rendition of the "Colors" album-- it was quite insane...