In Sweden you have to follow swedish law if you target your business towards swedes. If you use swedish language, sell directly to swedes and so on. I'm sure Germany has something similar.
Swedish is a bit more rare in International linguistics, but German is not. There is an entire population in the American mideast that learn and speak German as just an example of a very far off population of non-German speakers of the language.
Even without speakers, Gutenberg hosting these books does not mean they are targeting Germans. I can put any of these texts through Google translate and read them myself despite my only experience with German being one class in high school.
I can't think of a single US-specific website that has a German language option for German-speaking Midwesterners. There probably exist some, but their German language option will be very clearly described for this purpose when this is the motivation.
There's a difference between theoretical plausible deniability, and practical plausible deniability. This excuse falls in the first category, accompanied by other colourful excuses like "I didn't steal that, someone must've slipped it into my pocket and I didn't notice". Theoretically possible, but ultimately implausible and completely dismissible unless you've got proof to back it up.
This sort of "remote jurisdiction" is necessary. Otherwise the only recourse to criminal foreign sites targeting your country will be to endlessly try to block it, while blocks are cheap and easy to evade.
The GDPR rests on the same principle of remote jurisdiction. If your website targets EU citizens, you have to abide by EU privacy rules.
Even without speakers, Gutenberg hosting these books does not mean they are targeting Germans. I can put any of these texts through Google translate and read them myself despite my only experience with German being one class in high school.