Call async function on main actor from synchronous context

I am trying to understand as to why following piece of code throws an assertion. What I am trying to do is to call asyncFunc() on main thread/main actor from call site. I don't want to decorate asyncFunc with @MainActor as I want the function to be thread agnostic.

func asyncFunc() async -> String? {
     dispatchPrecondition(condition: .onQueue(.main))
     return "abc"
}

func callSite() {
     Task { @MainActor in
          await asyncFunc()
     }
}

My understanding was that Task { @MainActor ...} would execute all the following code on MainActor/main thread.