Eh these kinds of runs pop up all the time in the speedrunning scene. Busting fake runners is an easy way to make views on YT.
First signal is when it's from a person who doesn't have a history of consistent achievements on the public leaderboards. It's not a rule - sometimes there are extremely talented up-and-coming legit runners who go from pretty bad times to WRs in a matter of months (e.g. Derek MacIntyre in Factorio 100%), but their past runs often show steady improvements. Many fake runners pop out of nowhere, or have few/inconsistent top times/PBs.
Second, every serious speedrunning community has people who check the video recording frame-by-frame; other rules usually include a whitelist of mods, posting the save, (for games that have RNG) the seed, and (for games that have replays) the replay. Breaking the rules just outright disqualifies a run. The frame-by-frame videos usually are enough to uncover cheating. E.g. in Minecraft any%, stronghold navigation relies a lot on luck and intuition; but there are visual cues for which room might lead to the end portal, and if a runner doesn't take the split second to look in that direction it's a tell-tale sign they've already scouted the seed.
But yeah, some formerly-respected runners actually put a lot of effort into cheating, and it takes another expert runner (or even the whole community) to bust them. More in-depth for Minecraft: https://www.youtube.com/watch?v=XoEQ8wtawPI