A mixup from too much cake...sounds tasty...
Yes, in general 'come over for six thirty' could mean eating at a later point, and would mean that in some circles. But it was specifically discussed that it would be an earlier dinner, so that you could leave by a prescribed end date--8:00. In what world is serving dinner at 9 with no explanation or apology an ok thing to do? Regardless of the reasons, this was really rude. You could've had to be at work by 9, and it would've been rude too!
It's one of those things that makes my blood boil, and just seems like such total disregard for others. The 'oh, tee hee I'm always late!' personality is annoying enough, but when someone has specifically given a time they need to be out of there..do they think it's just a suggestion? If it really was a time got away from her scenario, she should have been falling all over herself apologising! I'd have been mortified.