For me I think it is a bit more general than "just" the IDE.
For a start, it's painful to install the apps I develop on Apple devices: it's limited in number, so once in a while I have to delete the sample apps I installed. Also I need to "trust the developer" for each app, and apparently more than once (everytime I uninstall the app maybe?), and it requires going through an unintuitive path in the settings and clicking the wrong choice the first time because "allow" is in red (as if it meant "cancel").
Then it has to go through Xcode. I hate Xcode, but I can accept that people like it. My experience is that regularly, I have to clean the cache, then clean the hidden cache, then quit Xcode, go and erase some DerivedData, restart Xcode and hope it works. All the iOS developers I know are aware of this, so it feels like it's "part of the process"? For Android you can use whatever you please, to the point where you could even write your own IDE!
Signing is a pain during Apple development. You need a development team, sometimes it doesn't work for some unclear reason.
The Xcode project files are a big mess. Developers routinely version a whole hierarchy of file (xcodeproj, xcworkspace and whatnot) without understanding what's there. You want to change something? Go to the Xcode GUI, change some obscure field somewhere, and your xcodeproj files will change in a way you can't understand anyway. "Trust me bro, it worked on my computer", I guess?
Compiling without Xcode is impossible. For Android you can just use gradle (or another build system if you really want), for Apple you have to use Xcode. For scripts or for a CI, you probably want the command line; I find the Xcode CLI extremely bad (I am a Linux person, my favourite kind of UI is CLI, so it's not like I'm confused with CLIs in general).
Talking about CI, you need a macOS machine. That is a pain in itself.
You want to share your app just with a friend? You have to go through TestFlight, send them an invite, and the app will only be available for some days (is it 30 or 90 days?).
Did we talk about SPM that chose to abuse GitHub for distribution? I cannot distribute just a binary, I have to dedicate a repo for that. Wanna use a monorepo? Your Package.swift cannot go into the "swift/" subdirectory, no no: SwiftPM WANTS it at the root of the repo. And SwiftPM will make a complete clone of the whole repo, so all your users will have to download the whole monorepo. People love to hate the Java ecosystem, but Maven solved the package distribution decades ago.
You want to do something with USB? Forget it. Really (don't get me started on MFi). Apple only supports a handful of devices, versus the plethora of Android devices. But still the Android experience is vastly better there: USB has just worked since the very beginning.
I don't know SwiftUI well enough to comment, but the article seems to confirm that it is absolutely consistent with the rest of the developer experience.
And for all that joy, you have to pay 100$/year?
I understand that developers must develop for Apple, because people love their Apple devices. I just don't get it when developer don't hate the Apple developer experience.