In Linux they would have been separate packages, but I don’t think the App Store has dependencies so how else are they going to get the required packages installed for their ide.
[1]: Yes, I know about WSL. No, that doesn’t count.
And this is not because no one wants to write a backend to target iOS, it's because Apple won't allow it. If you want to run native code on iOS, it must be built and signed with XCode on a Mac.
I will still note that debugging and monitoring are usually not going to work cross-platform - try to log to journalfs from a Windows system (wihtout using MinGW or similar).
if you want to build ios apps installing xcode is a requirement (to get the proprietary cli tools) but launching xcode is not required
to build native mac apps, neither installing nor running xcode is necessary, as you can build with the smaller cli-only tools which include only mac os headers/libs
As for those IDEs, they are a subpar experience of what the whole AppFrameworks are capable of, just like Borland of yore always playing catch up with Windows SDKs.
That's a bad analogy IMO. GNU/Linux is mostly POSIX compliant. There are some edge cases one runs into with C programming, but if talking about shell scripting, sticking to POSIX is definitely more portable. Bash would be a chrome equivalent, otherwise it's like targeting a subset of what every browser can do. Testing against Busybox/musl (Alpine) is a good way to assess portability.
Or asynchronous network programming with certificates, while using only POSIX APIs.
Dealing with filesystem capabilities, file locations, daemon configurations, port management utilities,...
Ah, and anything GUI related is not even part of POSIX.
I don't think that's true even if you consider IDEs that target the OS X frameworks only. Intellij has a product, Flutter targets iOS, I am not an OS X developer but those two jump out straight away.
As for developing on Mac - it's a Unix & so a very capable development tool to target a range of platforms.