The user is expected to make linear movements to operate a rotary control. This is the reason that most properly-designed applications utilize linear slide controls for similar functions.
I hate this UI paradigm. There are a bunch of graphic design apps on iOS that do this (e.g. Affinity Designer) where you can change something like line width. The icon shows a circle filling up as you change the property (it’s rotary so it’s the circle “ring” filling up, not the disk). Except it expects you to move your finger linearly to change the property, not in a rotary fashion so it’s very confusing. Also you can move your finger linearly up and down OR left and right. These movements are not exclusive of one another so you can move your finger in a rotary pattern and then wonder why the line width changes seemingly at random.