Apple will reject apps created from app generation services like Xamarin in 2018

It looks like it mixes true and false information. If it turns out the content is actually genuine, in spite of imperfect English, the underlying meaning could be that Apple will reject template-based apps generated by frameworks mentioned, and not that all apps created by them would be banned. This seems the most logical conclusion. Apple definitely wouldn't disclose such a huge policy change via one small letter to one developer, it makes no sense at all.

Regardless of the ethics of apple’s decision here, iOS’ market share makes it pretty clear it’s worth it. Does it not?

That’s fake news. There’s nothing to discuss here.

Depending on which stats you look at, iOS has around 12-16% of the global market share for smartphones, which is likely too small to invoke antitrust laws (which are there to discourage anti-competitive behaviour from players who are "too big to avoid").

None, regardless of what geek think they know about monopolies, what Apple has isn't one according to the law, so they are free to do whatever they feel like on their own platform.