The publishers claim doesn't hang on you rewriting their content. They say you've "[copied] the precise selection, structure, organization, and depth of coverage in [their] textbooks and mapped in substitute text, right down to duplicating [the publishers] pagination", and that you "[took] hundreds of topics, sub-topics, and sub-sub-topics that comprise [the publishers] textbooks and copied them into Boundless texts, even presenting them in the same order, and keying their placement to [publishers] actual pagination"; also, that you "[copied] or [paraphrased] with respect to the substance of hundreds of photographs, illustrations, captions, and other original aspects of [publishers] textbooks".
Look, most of us on HN have read many college textbooks. I think we all recognize that way more goes into a textbook than just the prose.
I'm not judging you, just message-board-nerding your comment here. This is pretty common in threads about legal actions. "We didn't do XXYM", where /\AXX..\Z/ is what's been alleged.
Allowing Smokey the Thermodynamics Bear was a super bad idea, by the way.