Looking back at the detail of the article it does seem unlikely that the associates (the article specifically says there are two) have that good of a recollection of the story Comey told them. Although it does seem like Comey told many people about these things and if he was as suspicious about Trump as he claims, it would make sense to broadcast this pretty regularly and broadly to add to his credibility later on.
That said, I don’t see why it is extremely unlikely that Comey loyalists had access to the memo and leaked it on May 11th independent of Comey. That scenario seems more likely than Comey lying under oath about his motivation for the May 16th leak. What would his motivation for lying be? Or is your theory that he has been lying a lot and did a good job except for this minor thing?