There is no such thing as 'confirmation' of the worst movie ever. It's all down to personal opinion.
You will notice that it is always the most critically-lauded, popular and financially successful movies that get these comments...never the really, truly awful movies that most of these people will never see and will never hear of.
But the sort of people who use that phrase do tend to be vastly over-opinionated and ignorant in their responses.
They will never make it clear they are stating a personal opinion, they will state it as though it is a fact.
I never really understand why not liking a movie has to translate to the 'worst movie ever' Truth is a lot of it comes down to their expectations of what they think the movie should be.
They read and believe the publicity that praises the movie to the hilt, and end up very disappointed. And not because it is a bad movie, but because it does not (and indeed could not) meet such lofty expectations.
And sometimes you can even see it coming...with posts on here saying 'The trailer for X looks awesome!', and 'OMG, I can't wait for X!.'
Most of us are savvy enough not to believe what we read and hear, and tailor our expectations accordingly.
I think a lot of it is borne out of a sense of inferiority. If I watch a movie and don't like it, even if it is a popular, very successful movie I never feel the need to post derisory, mocking comments about what on earth people see in it etc. I may indeed give my opinion, but it will be a considered one.
But a lot of people actually think that inwardly there is something wrong with them for not 'getting it', when everyone else did. And they feel the need to kick against it, to defend themselves against what they percieve to be a feeling of being made to feel like an outsider, of being apart from the crowd.