>It's a summary. if you put up a web app where you modify open source software as part of your stack, you have to publish those changes if people ask for them
Only specifically your changes to AGPL code, but otherwise correct.
This is reasonably well described on GNU's own page[0] on the AGPL.
Well I shouldn't have to publish them, if I'm not distributing that software.
Your private forks are private, even if you produce something using them. You can't taint that production or force someone to publish changes they don't want to publish and aren't distributing.
You are saying if a textbook publisher modifies open source layout software then uses it to set a textbook, they can't retain those internal modifications if someone asks to see them. I think they should be able to keep them private.
Only specifically your changes to AGPL code, but otherwise correct.
This is reasonably well described on GNU's own page[0] on the AGPL.
[0]: https://www.gnu.org/licenses/agpl-3.0.en.html