In this case, em dashes are doing something similar to a pair of commas, which can also denote side info but they do it more casually, and parentheses. "and parentheses" what? And they are also like em dashes? Edit: Thank you for the explanation folks. For whatever reason I found the use of commas to denote the aside very difficult to parse. I think maybe it's because of the first comma after "In this case."
Today's punctuation rules may have awkward beginnings, but the rules are standard, familiar, and subconsciously expected. This actually improves the speed at which I can read. The author's style had several places that were difficult for me because I have been mentally trained on one set of road signs and meanings.