It's not "advertised" as a replacement for Twitter; the word "Twitter" does not appear on the Mastodon homepage. It's talked about because at the moment there aren't many similar alternatives.
just because hacker news has usernames, comments, and voting, that doesn’t mean it’s “trying to be reddit”
there are countless little things one can change that make it an entirely different experience even if some features are similar.
just like hackernews to reddit, the experience i’ve had on mastodon has been a noticeable improvement over twitter. yes, there are comments and voting in hackernews, but the experience is noticeably different—and better—than reddit.