You could just as easily say that all dictionaries are prescriptive by design. In fact, they're both, but some lean more towards description and others more towards prescription. Defining concerted as something done by one person is, in my totally not humble opinion, leaning too far towards simple description. Yes, of course language changes over time, but letting the ignorant (e.g., people who don't know what it means to do something in concert) determine the nature of the change is just silly. Why bother having a dictionary in the first place? Just publish a piece of paper the size of a Post-It note with this advice: "Want to know what a word means? Just ask a couple of people at random. Whatever they say, that's what it means."

I'm riding my fixed gear bicycle as I type this.