It was a bluff designed to keep Comey intimidated...and quiet. But it wasn't very well thought out.

Think. If there WERE tapes, and Comey was telling the truth, they would never come to light. If Comey was lying, they would be held back in reserve and produced at just the right moment to demolish Comey's case. The last thing Trump would want to do is give himself away and let his opponent know he had a solid defense.

Either way, if there were tapes, it would be stupid to give away your advantage to Comey. But Trump's not a thinking man, his first instinct was to shoot from the hip, to try and catch his opponent off-balance and intimidate him, rattle him. Now why would you tell your enemy you had tapes when you didn't, or if they only contained evidence against you?

Like I said, Trump's not a thinking man.