Apparently you’re an extension developer, and you don’t know what it means for an app to run in background.
Read the article.
“On most, if not all, smart phones available you can have multiple apps running at once. When you have an app running, but it’s not the focus on the screen it is considered to be running in the background.”
Closing the app has nothing to do with running in the background, to be able to run in background, it simply means your app is able to bypass the doze mode on its own.