The author seems under to be under the wrong impression that Mastodon servers are isolated communities (and therefore compares them to subreddits). This is incorrect. Mastodon is a platform where a server of your choosing hosts your account, like an e-mail provider hosting your inbox. From that account, you follow any other account on the network, regardless of which server hosts that account. Everyone you follow appears in your home feed, and everyone who follows you receives your posts in their home feed.
It is unfortunately a common misunderstanding, and I must admit we didn't help the situation by leaning into calling servers "communities" on joinmastodon.org in the past (not anymore). There do exist plenty of "themed" Mastodon servers, aimed at specific communities, e.g. phpc.social which describes itself as a server for the PHP community, because some secondary discovery features of Mastodon lend themselves to what kind of feels like a slack/irc/subreddit, but ultimately, in their primary purpose, servers are service providers just like GMail/Outlook/Fastmail are for e-mail.
It is unfortunately a common misunderstanding, and I must admit we didn't help the situation by leaning into calling servers "communities" on joinmastodon.org in the past (not anymore). There do exist plenty of "themed" Mastodon servers, aimed at specific communities, e.g. phpc.social which describes itself as a server for the PHP community, because some secondary discovery features of Mastodon lend themselves to what kind of feels like a slack/irc/subreddit, but ultimately, in their primary purpose, servers are service providers just like GMail/Outlook/Fastmail are for e-mail.