How would your code look like if you wanted to fire off two concurrent requests and then consume their results later on? In C# it is just

var user = service.GetUser(id);

var promos = service.GetPromotions(category);

var eligible = GetEligibility(await user, await promos);

I guess like this:

    var user = try alloc.create(@Frame(service.GetUser));
    user.* = async service.GetUser(id);
    defer alloc.destroy(user);

    var promos = try alloc.create(@Frame(service.GetPromotions));
    promos.* = async service.GetPromotions(category);
    defer alloc.destroy(promos);

    var eligible = GetEligibility(await user.*, await promos.*);

but the claimed difference is not present in this code. It is that GetUser and GetPromotions do not themselves know whether they are async. The author of service is able to just take some readers and writers or some types or objects implementing some protocols and use them, then service's methods inherit the async-ness of its dependencies.