I don't see a good technical solution here, but I see two social ones: a) developers of FLOSS must understand they're writing software that can and will be modified and re-released by others, sometimes individuals and sometimes organized groups (like people making Linux distributions); and b) users need to understand that if they have a problem with their software, they should reach out to people from whom they got that software for support. If you got a program from developer's home page, you should contact them directly. If you got it from your distro, then distro maintainers should be your first line of support.
(Also c), some users won't understand b), and forwarding support requests to appropriate people is just part of publishing FLOSS.)
If themes are so important to you, pick your app based on that criteria. If you think an app is garbage because it doesn’t theme well, fork it and write a better version yourself.
That is an unfair inference because the author of the post explicitly says otherwise. To the extent he is speaking to you, he is only saying "thar be dragons" and that he will not explicitly support your use of a theme—therefore you should file any theme-induced bugs with the theme author and not with him.
I understand that the devs do not want to be responsible for bugs stemming from broken themes, but this is the wrong solution. Ultimately the solution is a better theme API, not this which would essentially stop custom themes all together for most end users.
In other comments people argue that this isn't aimed at end users, but end users would of course also be impacted from this.