So here's the thing, I don't think a textbook author going against a purveyor of online courseware has much of a chance, nor do I think it should have much of a chance, because it probably lacks meaningful proof that their works made a contribution to the creation of the courseware. Would I feel differently if the textbook author could prove in court that a substantial amount of their material contributed to the creation of the courseware, and when I say "prove" I mean they had receipts to prove it? I think that's where things get murky. If you can actually prove that your works made a meaningful contribution to the thing that you're competing against, then maybe you have a point. The tricky part is defining meaningful. An individual author doesn't make a meaningful contribution to the training of an LLM, but a large number of popular and/or prolific numbers can.
You bring up a good point, interpretation of fair use is difficult, but at the end of the day I really don't think we should abolish copyright and IP altogether. I think it's a good thing that creative professionals have some security in knowing that they have legal protections against having to "compete against themselves"