I agree, it was not paranoia. Hemingway was recruited by the KGB and was eager to offer his services. The only reason he didn't last long as a Russian agent is that they ended up not finding him very useful. [1]

He thought the FBI was following him because he knew they had good reasons to.

[1]: https://www.theguardian.com/books/2009/jul/09/hemingway-fail...