I don’t think ‘known’ really qualifies as a “weasel word” in this context. To me, “weasel word” is a somewhat wishy-washy or putatively unnecessary qualifier de

I don’t think ‘known’ really qualifies as a “weasel word” in this context. To me, “weasel word” is a somewhat wishy-washy or putatively unnecessary qualifier designed to weaken a statement in a way that allows the statement to be phrased in a way that seems much stronger. “Dozens of supporters” meaning “25 people” is weaselly; “virtual unanimity” instead of “99.9% support” is not always weaselly (though it could be depending on the context); “largest known in the observable universe” instead of “the largest we’ve happened to spot in a telescope” is not at all weaselly. In this context: “are you sure it’s the longest long-exposure in the world? Have you checked every single camera or device that employs a camera obscura technique?” isn’t a very reasonable objection. And “known” isn’t a weasel word so much as a tautological qualifier. It doesn’t weaken the statement in anyway, it just clarifies that the author is not ominiscient. If the headline was “believed to be one of the longest” because they were aware of a longer long-exposure, that would be much more weaselly.