When LLMs are used to write academic papers, then there is definitely a stronger requirement on the authors deploying them for identifying the source of ideas, as a matter of professional ethics (not law). But this isn't the issue being discussed, and it's not anything new with LLMs. Long before LLMs, academic authors occasionally got ideas from other places StackExchange without citing them properly in papers. (They frequently learn things from StackExchange, but most of the time they don't need to cite them because it's part of common knowledge. Cases where they got novel, cite-necessary insights is much rarer, as it will be for LLMs.)