There's another more thorough version from the same author, for what it's worth. Just moved on from that company but the overall points are the same https://diataxis.fr/
But they sort-of do, no? The left hand side bar has the top-level (bold) headers "Home", "Start Here", "Applying Diátaxis", "Understanding Diátaxis" and "Colophon". The middle ones could well be labeled "Tutorial", "Explanation" and "Reference", but that would be very confusing for the reader!