You will notice that every keypress in evtest is also preceded by a KEY_MSC. This is just the evdev responding to a scancode. If the hwdb has this scancode pre-assigned in 60-keyboard.hwdb then it creates the EV_KEY event that is mapped. What you want to do is turn this EV_MSC into an EV_KEY by mapping the unknown scancode to a evdev event code using the hwdb file.
The arch wiki has a good article on how to map these scancodes and identify what keycode you want to map. It is generic enough that it should work for most distros. Read all of section 2, it goes into specific detail about your question.