How do you know that Grassley didn’t find out about it from other sources?
because this FBI created document was sat on by the FBI until 2023
No-one else knew the content of this doc, or if it even existed. The person interviewed knew what they said, but as we know, the FBI “summary” might say anything, or nothing, or not even exist.
Republicans found out about the doc through “whistleblower disclosures”. Exactly what those “disclosures” were is known to only a few people.