Steve Jackson must have a soft spot for chess variants. Although Steve Jackson Games doesn't really have a very extensive line, and what it does have tends to be stuff that beats a game theme to death (Munchkin, OGRE, Illuminati, GURPS etc.), SJG has no less than three different chess variants for sale.
Or does it?
Tile Chess and Knightmare Chess are clearly chess variants. Both use the rules and pieces of chess and just add some new twists.
Proteus, however, while it appears to be a chess variant, could arguably be something quite different. It's a different sort of abstract game that merely borrows the moves of chess pieces.
The usual rule for judging whether a game is in the chess family is the knight move, which is unique to chess. Proteus does have the knight move, so it can be considered a chess-family game. And it does use the same board as chess. And, of course, it uses many of the same pieces of chess and their symbols, although also adding one new piece. In all these ways it looks like a chess variant.
But the game plays a lot differently than chess, not the least because the object of the game has been changed. There is no King in Proteus and therefore no check or checkmate. Victory comes from scoring the most points. One can also win by forcing a situation where the other side can't move, but the main way to win is by attrition.
This difference in objective, as well as the rotation of pieces between identities, makes for a very unchess-like game. I think it's different enough, in fact, that I'd hesitate to call it a chess variant at all. It seems to be an abstract strategy game that borrowed some moves and nomenclature from chess, instead.