He can feel confident because Facebook has a history of misleading users or oversharing data and this isn't a court room, rather the internet, and there is little consequence if he's wrong.
I'm not 100% confident that Facebook's intention was to mislead but the simple fact that these posts aren't being shown to the user(s) that they are being linked to is pretty damning.
I can be confident because Facebook spends more time and money studying user interaction with their advertisements than any other single aspect of their business. They have focus groups, they study eye movement with iris tracking, and they are leading authorities in online marketing. They don't make a change to their online ads without understanding 100% of the variables.
And, for the record, this analysis of Facebook's investment in user study isn't mere conjecture. I've been lucky enough to attend a talk by Facebook employees on their efforts.
How can you be so confident about someone's intent?
I disagree with your assessment but didn't downvote your comment because I want to understand.