I appreciate all the feedback. After another day to consider the situation, I'm slightly less frustrated as I understand this was beyond Marriott's control.

With that being said, I still hold to the statement that the "Guarantee" isn't accurate. It's great marketing, but it's deceitful as well. I've had 2 other occasions (random conventions, NOT big events) that were in the area and thus they weren't able to offer the Guarantee, YMMV obviously.

I know Marriott's marketing team won't like it, but it's time to change it to "enhanced availability" like the other poster said. Either it's a Guarantee, or it's not.

I've written their customer service, and expect to hear back shortly with a form response pointing me to the paragraph which says they have the ability to waive the guarantee as they see fit.