It's not clear exactly how Microsoft will detect inactivity, but it's possible the company could use Windows Hello-compatible machines or detect idle activity and lock the machine accordingly.
My monitor at work has the ability to detect if someone is sitting within a certain distance from it. If no-one is in range then after a couple of minutes it would automatically turn off the screen. The distance can be defined in the settings.
I'm sure there are a small number of people on here (who will likely comment) whose working patterns means that this wouldn't work for them - but it seemed to work for the 500 odd people we have on the floor.