Speaking on behalf of Medium, the post you are talking about was not Boosted in any official sense. As someone noted in the comments, the Boost comes with a notification and a badge in your stats. We don't Boost stories that are about Medium itself so your post wouldn't have qualified for an official Boost (as we've explained in our guidelines).

What happened here instead was simply that the post was popular among a group of people who are hungry to read about Medium.