Well first they need to proof that Viktor was actually copyrightable. If it was largely written by an llm, that might not be the case? AFAIK several rulings have stated that AI generated code can not be copyrighted.
That said, the article says "Okay, prompts, great. Are they any interesting? Surprisingly... yes. As an example workflow_discovery contains a full 6-phase recipe for mining business processes out of Slack conversations, something that definitely required time and experiments to tune. It's hardcoded business logic, but in prompt instead of code."
So the article author clearly knows this prompt would be copyrighted as it wasn't output from an AI, and recognises that there would have been substantial work involved in creating it.
Sure, I didn't have a license for the code that I read. But I'm pretty sure that doesn't taint my coworker's clean room implementation.