Trump self-servingly denying a thing isn't a refutation. Trump claimed that despite having an "all-time great" memory he couldn't recall the reporter and wasn't mocking his disability.
"Donald and I were on a first-name basis for years," responded Koveleski, who regularly covered Trump for the New York Daily News from the late 80s into the early 90s.
Trump's own words about the reporter (until he needed the distance) don't support Trump's denial. In fact, right before he launched into the impression he said, "...the poor guy...you gotta see this guy" as noted in several of the papers covering the dust up.
Since then, some supporters have come up with a couple of videos where Trump, mocking Cruz and a member of the military he took exception to, used exaggerated body language as part of the mockery. I've watched both. They're comparatively mild, but not so much that you can't credit Trump with the impulse to interject a physical routine in making fur on those who oppose him.
That said, the attempts at denial coupled with the language that preceded it, the knowledge he surely possessed, etc., speak to a different kind of exception here. Trump could have said, "I do that with all sorts of people when I'm making fun of them being ridiculous." He didn't. First, I suspect he didn't because it's not something he normally does. It's something that people had to work to create as an impression of normalcy and both examples are, again, appreciably more subdued, if related. Secondly, I don't think Trump was sure about why he did it. So he simply made a blanket denial and then went overboard on the don't know him bit to shore it up.
But, all that said, it's possible. It's possible that he was simply acting immaturely in a largely unprecedented level. It's still a black eye and a needle in a growing haystack of Trump self-aggrandizing and distortion.