Why would anyone ever believe what an intelligence agency says? This would be akin to listening to a snake oil salesman about the qualities of snake oil.
EDIT: My point is much larger than whether or not they wiretapped Donald Trump. My point is that what an intelligence agency says about what it's doing is completely irrelevant because it has very low correlation to what they actually do.
Suppose it is true that they wiretapped him. Then Trump has every incentive to prove it so, at huge risk of embarrassment to them. They could easily say "no comment" and nobody would blink an eye. So why actively lie about it?
EDIT: My point is much larger than whether or not they wiretapped Donald Trump. My point is that what an intelligence agency says about what it's doing is completely irrelevant because it has very low correlation to what they actually do.