Who else could claim to know what was said besides Comey ?
Only Comey would know. But that doesn't address whether the leak of the memo occurred before May 12 or not. It is perfectly plausible that the timeline is as follows:
May 11. NYT writes article about January dnner with DJT. Source is interview with Comey himself.People are getting the May 11 NYT article and the May 16 NYT article confused. They are about different events. The May 11 article dealt with the January dinner with Trump. The leaked memo detailed the February meeting with Trump. The May 11 article did not cover material found in the leaked memo.May 12. DJT writes the "tapes" tweet.
May 16. NYT writes article about February meeting between DJT and Comey. Source is the leaked Comey memo, but the Comey memo wasn't leaked until after the "tapes" tweet.