It's quite possible that SEO-wise the site does not make the cut into top x Google results but still is findable and considered by ChatGPT when it does its searches.

Especially in a longer ChatGPT conversation or via deep-research or more agentic modes (e.g. "Pro").

ChatGPT spends quite some time and diligence on searching.

Great for content that is not hyper search engine optimized but still (or even more) relevant. It bubbles up.