I agree with you that the rule is not helpful and should be even shorter or removed altogether. However, there isn't consensus including the stakeholders and dev team to do that. While the reduction to 15 minutes isn't my ideal choice here, I do feel it is a clear improvement.

While you are right it does little about bots other than cause them to make a small recalibration, it does penalize humans less, even when voting before 15 minutes. For example, under the 30 minute rule, voting after 10 minutes would reduce curation rewards by 66% while now it will only reduce them by 33%,