It is a terribly designed website. The intro post points to the (top-)left menu, but that doesn't contain any links to pages. The maps are accessible indexed by publisher in the menu below the site title, which is in light gray over a light background and overflows into a "More…" option at the end.
This is very cool and I'm glad the author compiled this material, but the site reminded me immediately of one I worked on (I ran the digitization lab in the special collections library for 3 years) in college, around 1997.
The Whitman Archive at UVA is ... unfortunately fairly similar in content presentation, even after having been updated a few times over the decades.
Not sure I can agree, I think the archive you posted is quite well laid out and from a cursory browse I found it easy to navigate. The site in the OP is far worse.
Unfortunate in the 'allegation of theft of style' sense, or something more serious?
edit: 'hamburger' menu also not found on my side--index at bottom of post seen, but no maps found.
What am I missing?