I don't see why, least my understanding, you hit share chat, it creates a public link... google's robots discover everything public and index it. Seems to me like the same problem would happen if you generated a link to share on any platform, and burned it and never sent information to any platform. Unless googles indexing all whatsapp messages, but that would be a much bigger story.
Anyway point is blame IMO falls on either chatgpt for not properly configuring a robots.txt, or google for not following it.
sounds to me just like googles bots are finding them... could also just be maybe chrome or similar taking note when it finds a page and dropping a dime to google.