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  there that interests me is a set of student-taken notes that has been on the lecturer's website  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.