The article refers to ActivityPub-based "microblogging" by assuming that Mastodon is the only client application available for that purpose. It is not. Mastodon is certainly the most popular client application for that purpose, but it doesn't have to be. Other client applications exist, and a better or more popular client application could be created.
When the point of the article is to get people to comprehend that federated social media is not a "walled garden" --
People are using open, free Mastodon, but in their minds, they are still in a walled garden.
maintaining the notion that a single client application is the only way to read or create a certain kind of content is a big part of the very problem the article describes.
And the author seems to be aware of this:
Often, I hear about people trying to explain the idea behind Mastodon to someone, who is not on the Fediverse, they often explain it with e-mail. However, nowadays, people don’t even experience this “choice of service” even with e-mail anymore. They get their e-mail when signing up with google and that’s it.
GMail is not the only way to send and receive SMTP email. It's certainly a very popular way to do so, but you wouldn't describe a concern over people being blind to their choices of email providers (or, indeed, their ability to host their own email server) as
The current [GMail]-signup is only removing the confusion of users on first glance, because it either hides the server-choice altogether, or leaves them with a choice that is impossible to make at this point of their [GMail]-journey.
If the author, or anyone else, wants people to have a better understanding of the nature of federated social media, describing it wrong is not a path to that goal.