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.
looking at the stats over at coin.dance we can see that 98% miners are signalling for BIP141 at the moment and currently 630 blocks were mined with this version bit.
so i am going to go ahead and say your speculation is not going to come true.
but it is a good speculation though, and i like it being around. a couple of the news sites already made this. it helps the price stay at this level and give time to people in order to accumulate before the testing of moon prices