Something about this menu approach feels so clever but wrong at the same time.
So this menu felt very normal to me, but it may be off-putting to newer users of the web who are used to contemporary conventions such as slide-out hamburgers (not to be confused with slider hamburgers :-) ).
I'm gonna go with "it's a bad idea".
Generally when a user opens the menu they're trying to go to a different page.
But there are other issues like bookmarking or sharing the page with the menu open. People won't expect the result.
It's really not difficult to create an on-page menu so this seems more trouble than it's worth.
I think it is a beautiful, simple solution that just works (and also did work perfectly in the past when we didn't have JS)