When doing a Co-Op Run, the person who is not recording can easily use commands in chat without it being displayed to the OP, who is recording the run. Basically the other person could type /locate Stronghold and share the coordinates in voice.

Why is there no rules that everyone participating in a co-op run (or at least for duo and trio) has to submit a video proof?

The person filming must create the world, therefor showing cheats is off.

In the rules Cheats have to be on. for the video proofer. The person recording can see commands and probably can't spam F4+D fast enough to not, and chat must not be hidden stated by the rules (They can see that when you use /time set 0 at the beginning). But /locate is one command that doesn't show in chat for people who have not typed it. I would say the solution to this is to require to shown the server log after the run. I would also think that /locate Stronghold isn't that much of a cheat they need eyes for the portal. But if they haven't thrown any eyes than it is kind of obvious that there are cheating. I don't think people would try splice a Co-op run though.

I mean if they cheated like that it would be pretty obvious.

Theoretically you can check for the stronghold at the beginning from an island spawn, travel in that direction already and locate some villages on the way.

I don't think people would cheat on a co-op run, but I think that the category is not as competitive as it could be without some more rule enforcement.