> I didn't know that, which is why I thought "|| async move ..." was some weird form of pseudo-async-closure instead of what it is: a function that returns an async function.

It does not return an async function, it is a closure that returns a future. Carefully read the function signature I had posted:

    fn foo<F, Fut>(f: F) where F: Fn() -> Fut, Fut: Future

async move {} is just a future, there is no function call. || is a closure, put them both together and you have a closure that returns a future.

edit: I'm trying to think of how else to explain this. a future is just a state machine, an expression, there is no function call.

   let f = async move { };

Is a valid future, you can f.await just fine.