How does it not look great on mobile? It simplifies the number of options and simplifies the navigation accounting for the fact that mobile devices would require lots of scrolling on a tiny screen to see as much as you could on the desktop. On the desktop it uses all the screen space to show you more detailed sections you can click right to if you know what you already want.
My apologies. I forgot to whitelist the site for JS, and when I viewed it, almost the entirety of the page was taken up by the categories sidebar[0].
With JS it looks much, much better, and navigation is definitely usable. Though at this point it's raises the question of what is navigation: are all those item categories navigational aids, on par with other nav items? Or are they a product index and separate from navigation items? In regards to the GP's suggestion of "showing all links at once", I think it's fair to say that links that only go on some pages but not others don't count towards the "overpopulated navbar", which is usually intended to hold navigation aids that are available on all pages.
So while I think the site in question does a good job of navigation, I think it does so not by choosing a flat design with all links available in the nav, but by cleanly keeping the navigation to just a few key elements ("Contact Us", "Order", "Activity", "Log in") and deferring all other links to separate UI elements that exist only on the relevant pages.
