Note that Proximity Service does not prevent the device from falling asleep when the device is being used normally with the screen on and user activity timeout has been reached. However, the device will not fall asleep as long as the screen has been turned off by the proximity sensor as this effectively counts as continuous user activity.
Regarding incompatible devices - if your device is listed as incompatible with Proximity Service, turning the screen off on these devices enforces pausing of any foreground apps, so the usefulness of this app is significantly reduced. If your device has an AMOLED screen you may be able to find an alternative app that works for you if that app uses a black screen overlay to achieve the screen off effect.
This app is open source - follow the website link at the bottom of this Play Store listing for the GitHub repo.