I figured your link would be this one:

https://www.youtube.com/watch?v=JTfhYyTuT44

(Folding Ideas, "In Search of a Flat Earth")

He takes a couple of their claims seriously about what one will see when attempting particular experiments involving a very large lake, attempts them, sees the results one would expect if the Earth were curved, and reports this to some flat earth community forum, refining the experiment as they suggest ways he may have screwed it up, and continuing to find curvature (obviously).

The real story is how they react to contrary evidence delivered entirely on their terms, and where that community was heading four years ago (beware—I guess—that part also becomes necessarily "political").

[EDIT] I guess I buried the lede for this site's interest, which is that the video devotes a fair bit of time to how the Youtube "algorithm" took a little success for Flat Earth videos as a cue to aggressively promote them to people it identified as maybe liking them (those inclined to fall down that particular rabbit hole—which involves a lot more than just the specific belief that the earth is flat), but now flat earth is in decline, because that and other "algorithms" started sending the same folks to... Q-anon content instead.

Incidentally, there was a somewhat-big documentary on Flat Earth some years ago that included some folks from a flat earth convention trying some experiments very similar to the ones depicted in this video (involving visibility of objects across a large lake), with predictable outcomes.