I don't have an answer but...
My guess is that those types have @MainActor somewhere we can't see. Xcode interfaces don't show the entire truth. Like when you look at Task.init there is no way to know why it inherits the actor and your own closures don't! You need to look at the stdlib source code for that. So maybe the same is happening here. Maybe is not even annotated at the Swift level but somehow on the internals of SwiftUI.
Hopefully somebody can bring more light into this!
This recent pitch helps in some scenarios ([Pitch] Clarify the execution of non-actor-isolated `async` functions) but not in this one. But codifying more rules around this is good fur understanding. Worth looking into it ;)
I wish Xcode showed more things inline. Android Studio does this for variables and is very handy, specially since what we call "context" is actually an object in Kotlin, a coroutine scope, which lets the IDE show it like any other variable overlayed in source. Very handy.