I just tried it on their website, using the desktop browser, and the experience is absolutely OK: you just get the menu as in any web app, and you can close it to go back, etc. Just an old-school page which is blazing fast ... because it is an old-school page. It renders faster than a typical animation to open a sidebar.

I agree, but its not intrinsic to the approach of less JS and more pages.

You should of course not have a menu button on a desktop view. There is plenty of space to show the menu without hiding it behind a button.

Maybe it is you who are mobile centric?