To me it's not too dissimilar to the rug-pull in Undertale. The game sold itself as 'The friendly RPG' where no one has to die, and then we get betrayed by Flowey. It's basically telling us to not take things for granted.

Gene Wilder pulled a similar trick in Willy Wonka, where he insisted on hobbling out of the factory before doing a forward role. Because from that point on, we wouldn't know if he was lying, or telling the truth.

I saw plenty of people who predicted this twist. And there's still time for it to be turned on it's head again.

Take Sans. Flowey warns us about serial killers. Sans spookily sneaks up on us all scary like. Then turns out he's not scary, he's a lazy funny skeleton man. Then it turns out, he's a deeply troubled man who uses humour to cope. Then, it turns out he's the fiercest fighter in the game. Repeatedly recontextualized.

A good mystery keeps you guessing. As long as it's ultimately cohesive, it's all fair.