Lol... what? Tell me you don't understand Google without telling me you don't understand Google.
All that query does is tell you all the sites that Google has indexed under that address, which is obviously a small subset of all the links that exist.
You genuinely think they had a page with all the share links (which have random GUIDs) in a big list ready to be crawled? No, and Anthropic said as much:
[Anthropic] Spokeswoman Amie Rotherham added in an explainer that: “We give people control over sharing their Claude conversations publicly, and in keeping with our privacy principles, we do not share chat directories or sitemaps with search engines like Google. These shareable links are not guessable or discoverable unless people choose to share them themselves. When someone shares a conversation, they are making that content publicly accessible, and like other public web content, it may be archived by third-party services.”
So, these links escaped by other means. Does Chrome automatically send pages it hasn't indexed to Google when they're viewed (which would be dodgy, but also not surprising) or did people actually post them in places where Google could see them? It's simply one or the other.
It's not possible for Google (or anyone) to simply conjure up every single valid link - there's a reason GUIDs exist; they're globally unique.