I read the article you were talking about, and I didn't like it either. I love when people highlight my articles. It's valuable feedback. It shows me they're paying attention. It shows me which lines or paragraphs or whatever touched them. It tells me they're making an effort, even if it's not a very costly one, to reach out and communicate with me.
There's one guy I'm thinking of who highlights like crazy. He's highlighted my articles like crazy for years. I love him to death. Not because he highlights, but because by highlighting, he showed me something of himself, and he encouraged me to get to know him better.
If Medium has saints, he's one of them. Long may he highlight!