Starting with the confidence from the "Sanity Check" challenge, we initially didn't take this challenge to seriously.
After trying to find some hidden messages in the "rules" section of the Discord server for multiple hours, we gave up.
Who would've thought that the flag would only appear to the people who looked at the "rules" section on the website? Insanity.
Anyway, we found the flag on a moving image of a train behind the rule text. Insane.