Yes, it is essentially a formality. No miners signaling for BIP91 is going to be crazy or dumb enough to not actually follow through when they'd be stepping on their own toes.
While I agree, there is a scenario where a miner cartel could profit from reverting to non-BIP91: speculation. Just before the cartel moves out of BIP91, they could short sell a large stash of coins and they would be almost surely make a large profit from it. And then, at the bottom, they could simply re-buy, signal all for BIP148 (so Segwit finally locks in at August 1) and then enjoy the rise to da moon
They would lose a LOT of credibility however with this move. And the 2MB hardfork part of Segwit2x would be dead.