Affected are https://mastodon.social and https://mastodon.social/about. Linking to actual user content actually works, so unlikely a problem of user generated content violating TOS but Twitter pro-actively avoiding supporting the platform.
When I go to X department store, they'll happily look up how items are priced on their website for price matching, but they won't look at the website for Y department store to whom they would also price match. Is that anti-competitive?
That's a long way around to what level do businesses have to help you in sending you to a competitor?
Antitrust laws is not about waiting for a monopoly to establish itself and then deconstructing it, but about preventing one.