If you actually read through that they say theme away to your heart’s content, just please don’t report issues to the app developer, report it to the theme developer.
They say that lots of time they could spend developing is managing and investigating bugs that end up being due to the user installing some random poorly-made theme, wasting precious dev time that they are donating for free.
It’s a perfectly reasonable request, and has no bearing on whether an app is proprietary or not.
E: the guy above has drastically changed their comment so now mine probably doesn’t make sense.