Is it possible ... just possible ... that the NYT did not have any memo for the May 11 article and that the source was an interview with Comey himself?
It's possible that the NYT never had "physical possession" of the memo, just what was read to them, orally delivered.
They could have an e-mail, digitally delivered. The mail could be an exact copy of Comey's memo with complete headers, or excepts. Comey's name could me "masked" from an email, so NYT would have to trust its source.
I wouldn't say the physical possibilities are endless. But no matter how you slice it, Comey leaked directly or indirectly to the NYT, before the May 12 tweet.