What I'd like to know is why he chose Rails over another language, say Python for example.

I don't know why you're being downvoted, but I agree with you. For pages that I know will be long articles, I tend to scroll immediately downwards to glance at the first sentences of the first couple of paragraphs. This helps me decide if I want to invest the time in reading the article entirely.

The cognitive load you speak of isn't necessary bad though. In the article's example, there may be a pause in the user after the hover-over but that's because additional (useful) information is being presented.

I agree with your second point though I think it's very minor to have your cursor activate a hover-over effect when scrolling. When this happens to me, and it very rarely does, I just move my cursor. It's not something that bothers me personally.