I bought Toolbox and Ultimate Toolbox and NG's Mother of All Treasure Tables, as well as Dungeon Alphabet, and I completely agree that it's not a reference work.
I think that Treebore's POV represents an interesting group of readers: folks who were never familiar with DA prior to its book publication format. I knew DA on Michael's blog as well as in Knockspell #1, so I'm definitely coming to it from the POV of "hey, Michael added some tables to his interesting inspirational DA essays." Someone who wasn't aware of its previous incarnations, and who owns the aforementioned books, will probably think that it's pretty lightweight in comparison.
I see the tables as one example of ways to leverage the design ideas for each letter's topic; there are certainly a number of other topics that would go into an exhaustive dungeon design book, but that's not what DA is: it's an ideas-inspirer, rather than a comprehensive resource (in my mind, anyway).