Earlier quoted context omitted.
Any app should be ready to accept focus. It should never steal focus. When I start an app, and wait for it to launch, I am attempting to give it focus. I can withdraw that intent at any time by doing something else (e.g. switching apps) and the launching app should finish launching and wait in the background.
Your specific example is just repeating what I've said, but your generic point is wrong: let's say you have an important task in an app with unavoidable interruptions, during which you switch to your browser to mindlessly scroll. You most likely want to return to the main task as soon as possible because that scrolling is just a time filler because you can't do anything productive. So you don't need any notification…
"you'd prefer to seamlessly be switched back to the main app to do the thing you launched it for"
Would I? How do you know? Why do yo think so? This is such an odd thing to be sure about.