> I know of exactly zero other social media platforms that ban the mere mention of other social media platforms

WhatsApp banned links to Telegram.

Facebook ranks links to Twitter/YouTube lower than content without such links.

> Precisely none of those companies are owned entirely by individuals claiming to be "free speech absolutists"

Either Elon Musk is no longer "free speech absolutist" or does not consider Twitter policy must match his personal preferences.

> Mastodon is not a company, major or otherwise

Competition may be not only for money, but also competition for users. It is quite possible that critical number of users will leave Twitter for Mastodon.