A.V.
My problem with that party post is that there didn't seem to be any "warm up" activity. Those that abided by the 1PM guideline found themselves sitting and waiting at the bounce place, and then the mother was unclear about the end of the party, too. That's why it was rude, not that she had a big activity start later. If I expect my kid to be at a party 1-3, and it's really 1:30 to 3:30, that matters. Don't lie to me on the invite.
While you take time to get ready, the OP's point was that this wasn't get ready time. This was a deliberate "I didn't like how late people were last time so I'm going to fake it this time" move. Which is rude. She could just as easily have said, "Bouncing starts at 1:30. Please arrive by 1:15 to sign waivers and be ready to bounce!" Whoever didn't arrive on time missed some of the bounce. That shouldn't have been the timely parents' problem.