The AMS is trying its hand at curation as well. The project shifts some of the work onto authors and seems most useful for undergraduate subjects at the moment, but the names behind it should help.
I note that slthough it mentions textbooks, it says this:
"They have not been published elsewhere, and, as works in progress, are subject to significant revision."
So I understand the model behind these materials this to be that in the end the goal is to publish with a publisher, not to offer the material for Free download, and that these works are being developed, and welcoming feedback during that process.
I don't believe OP's page has that model. I think OP's page is works that the author considers finished.
For example, one item [1] there that interests me is a set of student-taken notes that has been on the lecturer's website [2] for seven years. My prediction is that it will never be changed or fed to a publisher. I think that in trying to avoid conflict with publishers and blame for unstable and error-filled texts the sentence you quote saps the enthusiasm of the visitor.
[1] https://www.ams.org/open-math-notes/omn-view-listing?listing...