Well, if he did, then he wouldn't be handing out any pardons. I'd go so far as to say that if impeachment proceedings were begun, and things weren't looking good for Trumpty, he'd resign, as Nixon did, and expect Pence to pardon him.
But were that to happen, say, next year, Pence would be running. There's a good chance he'd end up the default Republican nominee, depending on the timing.
And he'd lose. So winning surviving till 2020, and then winning, is very important for Trumpty.