"But how is it their fault that app developers don’t shun them for Android-only, or Windows (do they still make a phone OS?)"
Antitrust enforcement isn't really about "fault". It's not there to punish, it is there to force markets to be more competitive.
Saying that people can develop for Android, so this isn't monopsony/monopoly is sort of like saying Standard Oil wasn't a monopoly because people still could use coal and steam power rather than Standard Oil's petroleum.
While many things about iPhone and Android phones are similar, there are also a huge amount of differences that put huge barriers to going from one to the other.
Network monopolies are complex things, and they aren't about people being able to charge "whatever they like." They are about significantly reduced competition and significant barriers to competitors. Not everything has to be so black and white.
"Apple is like Tesla."
Not really, particularly not in the respects that this article is about. For a consumer, going from a Tesla to another EV is not a big deal. Also, EV's are in their infancy, and antitrust enforcement tends to cut some slack when that is true. Tesla could (very reasonably) end up being forced to separate their car business from their charging network when they get big enough.
"They aren’t Facebook. Nobody has to buy an iPhone to keep in touch with all their friends who have iPhones."
This isn't what the article is about. This is about the developer side of things, not the consumer side. Developers spend huge amounts of resources writing code that runs on only Apple devices. A very large number of users have iOS devices, not Android devices, and they only way to make apps for them is via the Apple store. Yes you can choose to develop for Android, but aside from the cost of switching, you are now selling to a completely different set of people.
For another analogy, imagine you're a clothing manufacturer, but if you want to sell clothing to women, you have to sell it through a single retail store chain. Saying "you can always make clothing for men" isn't particularly helpful.