Sludge metal isn't the most clearly defined genre/sub-genre to begin with, but I'd say that it's kind of its own thing on the basis that it took strong influence from music external to metal and that it existed when doom metal was still relatively young. However, a lot of the riffs are basically straight-up doom metal riffs, so I could go either way, provided we're not talking about modern "atmospheric sludge" or Mastodon-style stuff.