Dave didn't seem to mind cashing the checks he got for all the "agile" books his company published. This piece would have been interesting in, say, 2008, but now he's just riding the wave.
From what I can see none of his own books are actually about "Agile". In fact "Agile development with Rails" is probably the only one that used that word in the title, and I can't really fault him for taking advantage. Also he wrote some quite good books, 'Pragmatic Thinking and Learning' come to mind. I don't think your comment is particularly fair, nor does it diminish the point he is making in any way even if you'…
As to your point about whether they wrote any books on 'agile' I think it could be argued either way. More specifically, whether or not "The Pragmatic Programmer" (the book they made their name with) counts as a book about 'agile' is an interesting question, and one that throws some light on the wider debate.
Although they sometimes talk about "pragmatic programming" as if it were a thing, in the same way as XP and Scrum, their approach was more one of giving a smorgasbord of potentially useful techniques and practices rather than codifying a tightly defined set of best practices.