"LLMs learn the same way a person does, it's not plagiarism"
This is a popular self-justification in the art-plagiarist community. It's frustrating to read because it's philosophically incoherent but making the philosophical argument is annoyingly difficult, particularly if your interlocutor maintains a deliberate ignorance about the humanities (which you already know they do). But there is a simpler mechanical argument you can make instead: "learning" is inherently mutual.
A teacher “learning more from their students” is such a common observation that it is a cliché. Colleagues mutually learn from each other in professional settings. Actual artists are in conversation with one another, not just learning from a static historical canon. Etc, etc.
LLMs cannot do this. The output that an LLM produces contains a sort of poisonous residue that makes it destroy the reasoning capacity of other LLMs; this is a well-known problem in the field, known as "model collapse".