Any person who's used a computer in the last twenty years would understand that the interface is not responding and try alternate input methods (like scrolling, one of the most common paradigms on web pages.)
It doesn't "just look broken".
"Did anybody else spend half a minute trying to interact with the "app", then give up and close the tab, thinking it was broken before coming back here to discover that it was just a full-page screenshot of a desktop app?"
xauronx, and the majority of the other folks explaining the issue, took a much more reasonable approach—"it was a .5 second instinct to interact with it and then I figured it out. Not sure if that constitutes a negative UI experience or not, but it IS an issue. "
It's just unbelievable to think that anyone familiar with a web interface would get so confused by the screenshot that their only recourse would be to abandon ship.