Cassette total width is given in the "standards" section. That's been easier/more logical to me, which is the main purpose of the site's existance (so I can find correct data wherever I am, whenever I need it). But your remark is welcome, guess it does make sense to keep it all in one place. My logic was - compatibility part is about cog spacing (pitch), for index shifting reasons.

Similar logic went to freehub compatibility article. Just stated what kinds of spacers are needed for different cassettes on different freehubs.