That paragraph continues. The letter is absolutely also aimed at end users:
> If you like to tinker with your own system, that’s fine with us. However, if you change things like stylesheets and icons, you should be aware that you’re in unsupported territory. Any issues you encounter should be reported to the theme developer, not the app developer.
Just not with the same vehemence as its message directed at distros.
But I mean it's also technically correct. How could an application developer make any kind of guarantee in the face of arbitrary theming and tinkering by the end user?
Read that quote extract again—"If you like to tinker with your own system"—that is clearly saying to the distro developer to understand the distinction between your personal preferences and what you might distribute to others.
If the letter was aimed at end users, that qualifier would have been redundant. Therefore the letter is not aimed at end users.
> If you like to tinker with your own system, that’s fine with us. However, if you change things like stylesheets and icons, you should be aware that you’re in unsupported territory. Any issues you encounter should be reported to the theme developer, not the app developer.
Just not with the same vehemence as its message directed at distros.