Handing back a scored paper would probably be one of the ways I'd test for a written plagiarism. If the student gets mad at me and fights then they didn't plagiarizer. It's the presumed nature of humans to want to fight back when wrongly accused of something--even more so when you're found guilty of something. I'd accept a rebuttal saying that's not a correct model for every student and as such I'd find it evenly as "not-suspicious" if the person went to my boss or the dean and asked for them to get the points back.

If the student does nothing then I'd assume they did copy.

I'm sure people will be mad about these assumptions but from my experience--and from what I've seen my colleges fall into after wrongly accusing someone of cheating--I'd definitely say this is an appropriate average model. I'd also say it's lived to fruition in this case. The author got mad and wrote a fantastic expose on this situation. If I was the professor I'd correct what was obviously my mistake at this point.