Global Actor annotation on class behaves differently as SE quotes

Using Swift
,
1

I found this sample code while dealing with Quick library

class SwiftBaseClass {
    func someMethod() async {
         print(Thread.isMainThread)  // false
    }
}

@MainActor class SwiftSubClass: SwiftBaseClass {
    override func someMethod() async {
        print(Thread.isMainThread)  // true
    }
}

according to Using global actors on a type section of SE-0316, This Code looks invalid.(superclass and subclass has different isolation rule) Is there any information about this behavioral change? Or is it just a bug?

my toolchain is Xcode 14.3.1.

and this is output of swiftc --version

swift-driver version: 1.75.2 Apple Swift version 5.8.1 (swiftlang-5.8.0.124.5 clang-1403.0.22.11.100)

Target: arm64-apple-macosx13.0