The President of the United States should not be playing a game of bluff via Twitter. That you think this is normal or applaudable behavior for our president is concerning-- not just because you personally may have questionable judgement but because it demonstrates the normalizing of the unacceptable behavior displayed by Mr. Trump.Why not. Maybe Trump's bluffing or maybe not. Perhaps Trump suspects Comey will lie to the press and is warning him there may be evidence to call out Comey as a liar. There's just not enough there to get the whole picture. I think Trump is AOK and is definitely for America. I see nothing wrong with his comment.
The President does not need to go on Twitter to warn a fired FBI director that there is possible evidence to disprove possible lies. If such evidence exists, and if Comey decides to lie, well, then the President can show his evidence at that time. There is no need to forecast such a step; the "warning" is unnecessary for someone who occupied such a position as Comey did. Trump comes off, yet again, like a petulant child with taunts that one might expect more from the schoolyard than the Oval Office.