According to the court's judgement (which you can read
here in English and of course
in German) there's no need to prove that the PG site targeted German users, merely that the books were still under copyright in Germany and had been made available there.
They go on however to argue that it
was targeting German users because:
- parts of the site are in German
- German language works are available
- the PG License bolierplate put at the top of every book uses the phrase "anyone anywhere"
- the fact that PG issues a disclaimer about checking local laws means they are aware users in other countries will download them
(see page 10 of the linked doc)
Anyway, those are the court's arguments.
I would think that if intent is indeed not a factor, merely availability and protected status, then the big issue becomes jurisdiction. The court seems to think not only can it claim jurisdiction but that it will be able to enforce it.
And by geo-blocking the site, even pending appeal, PG would seem to be lending some weight to that.