It was possible, but required you to author your own expressions.
As you can see, selecting Moving Average from a dropdown menu and choosing in how many steps back you want to go is a little easier. I just wanted to confirm you were doing it the hard way if you didn't have to.
As this challenge is easier solved in a more recent version of Qlik Sense (as has been posted), I suggest we put upgrading on my ToDo list. I will be back on your site next week. It should be fine to kick the version to April 2020 SP5 without much trouble.