Easy, he sees an R indicating party affiliation (strike 1), that even those in his party say he is guilty (strike 2). In other words he doesn't know for sure, but the fact that he is a conservative Republican, obviously conservative because other Republicans (GOPe) don't like him which is always a sure sign he is a bona fide conservative (strike 3), while the thought never crossed his mind that Halperin was capable of being a perv because he is a liberal through & through.
Or to state it more simply, he is biased, and his bias determined his findings. No trial or jury needed.
Good analysis!
To your excellent analysis, I’d add this: if Heilemann had admitted having known about Halperin, he would have been criticized for having worked with him for so long.