It's not particularly enforceable. Given that this is a casual jam, and not e.g. an academic certification or a jam with cash prizes, that doesn't really bother me. However the rule is there and on the stricter side because people do sometimes make it clear how they've used AI. The rule is there so it can be referred to when necessary.
As for why, I'm not really up for debating that, beyond making a few points: Gen AI largely lacks transparency, and intentionally so. Gen AI has a huge carbon footprint. Gen AI steals from creatives. Some gen AI companies also provide AI to the military to enable genocide. Using AI in any capacity normalises and contributes to that.
I have considered improving the wording of the rule, but it's very low on my list of priorities.