However I would not characterize that as "Poettering completely misunderstanding a perfectly valid use of su".
It seems a quite complex issue about a workflow that is only half-working even without systemd. A systemd patch has been committed that fixes the most blatant error, but of course it doesn't solve all problems since most of them existed before systemd.
I don't know in what corner of the universe was that "only half-working". It was working absolutely fine. It has been working fine since... I don't know, v7, when it was first introduced.
The few times I tried to run GUI programs through su I always got plenty of warning about DBus and stuff. But I don't do that very often, so that's all I know.
However I would not characterize that as "Poettering completely misunderstanding a perfectly valid use of su".
It seems a quite complex issue about a workflow that is only half-working even without systemd. A systemd patch has been committed that fixes the most blatant error, but of course it doesn't solve all problems since most of them existed before systemd.