I am no lawyer, but I think anti-trust is a stretch in this situation. If Apple were to remove Twitter, it would not be for the purpose of eliminating competition. It would be because Musk had destroyed the infrastructure and human capital of the company to such a point that Apple could not be assured of basic security and appropriate moderation.The "right" a company has can depend on how much dominance they may have in a particular market and the reason/intent behind the blocking or restrictions. Android and iOS control nearly 100% of the mobile OS market with iOS currently having around 55% share in the U.S. Apple not only limits app access (restricting alternative app stores, sideloading, etc.) on iPhones/IOS they sell but also those sold by third-party retailers, and even restricts what end users can do. Antitrust laws and regulations are meant to prevent dominant companies from using their position to unfairly control/influence activities in the particular market(s).
If Apple were to show that removal of Twitter from the app store is justified, it wouldn't necessarily be an issue. My main point was that companies having too much control of a market (like mobile OS) can be problematic, and why antitrust laws and regulations are necessary.
And, yes, moderation is absolutely an essential part of any platform. Apple has every right to be concerned about it when deciding who gets to be in their App Store. It's not only about hate speech, political conspiracies, and lies. How about a school shooter that posts a video that does not get taken down quickly because the moderation team has been decimated? How about videos and other content that exploit minors?
Again, I am not saying that Twitter should be removed, but Apple has every right to make this decision if Twitter lacks the will or staff to moderate content.