Google added Google NotebookLM to the list of Google crawlers, under the list of user-triggered fetchers. Google said they added this user-agent "based on feedback."
User-triggered fetchers are part of tools and product functions where the end user triggers a fetch. For example, Google Site Verifier acts on the request of a user.
Google added the Google-NotebookLM to the list of user-triggered fetchers over here.
It reads, "The Google-NotebookLM fetcher requests individual URLs that NotebookLM users have provided as sources for their projects."
Here is a screenshot:
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Image credit Lizzi Sassman