That BS's statement definitely makes clear that Demandred actually HAS a secret identity, i.e. is posing as a certain 3rd Age person (like all other Forsaken did as well, at least temporarily). If the revelation would just be that Demandred is the only Forsaken, who never had another identity but just has a wordly power base somewhere, his "identity" would hardly be the biggest secret in the WoT with a great pay off, as BS told.

That also fits with the report that "Sanderson said that we had seen Mesaana's alter-ego on screen as of Knife of Dreams, but that we had not seen Demandred's alter-ego on screen as of Knife of Dreams. Sanderson would not elaborate on whether Demandred's alter-ego had been seen in The Gathering Storm — Sanderson didn't want to narrow down the suspects for us if Demandred really had appeared in The Gathering Storm."

Especially the last sentence can only mean that Demandred might have appeared in his secret identity in TGS, because we saw him without a disguise in TGS. There also wouldn't be any suspects to narrow down, if Demandred doesn't have an alter-ego.