I know it makes the run unverifyable, but..........how?

For example, this run was rejected: https://www.speedrun.com/Obby_But_Youre_on_a_Bike/runs/mkq2px5m

With the reason being "Insufficient evidence (Note: Runs recorded using Roblox's recorder aren't verifiable)".

Although, I have now just read the rules, and it says "Do not use Roblox's In-Built Screen Recorder as it is highly innacurate (Free alternatives: OBS, Medal).".

But after viewing @SpeedyLuigi37's rejected runs: they aren't even inaccurate? Everything that happens in them seems to be clear?

Well there's a reason we're able to tell which runs are recorded with roblox's recorder. It's not guess work and its very obvious once you know what you're looking for. The primary issue is the way that roblox's recorder works. It doesn't actually record your screen or gameplay, but your player position. Based on that data it produces a video that in-turn is unverifiable. The player movement is very inaccurate especially when it comes to high speeds, and the timer is unreadable. Those two reasons are more than good enough.

I hope that clarifies things.