By contrast this sounds plausible (also from CB):

"Old and cynical party of one here to propose a theory:
Someone’s PR team leaked the dating stories ahead of the film festivals to garner some press. The same PR person likely tipped off the paps re: the posh dinner “date”, and probably planted the stories on the gossip sites that they’ve been an item since filming.
Jump ahead to the film festival, where the only good things being said about ISTL are the reviews of Hiddleston’s performance.
So, this film isn’t going to be a contender, neither actor is likely to be nominated for a major award. (Or a leg lamp, for that matter.) Therefore, no more reason to play the “are they or aren’t they” game, hence the flat out denials about dating. If the film were getting better reviews, I have no doubt that this would’ve played out differently."

Makes some sense of the U.S. Weekly article.