The point being made is that just as the search engine was the primary means for users to discover content yesterday, so the LLM agent will become the primary means tomorrow. And that content doesn't have to be in the training data, but if an agent is unable to access some particular content, then it won't be discovered by users. Similar to if a rewatch engine is unable to access it.
Would removing your website from google search results cause people to go directly to your website?