This is an extremely short (and misleading) summary of the courts argument. Among the reasons for assuming jurisdiction is the assumption that project Gutenberg targets users in Germany and part of that assumption is Project Gutenberg’s statement that the goal is to make the works available globally (thus including germany) (quote: “anyone anywhere”). The fact that content and at least parts of the page is available in Germany adds to that assumption. It’s been established that websites targeting german people (also) fall under german jurisdiction.
So no, not an extraordinary or new theory of international law.
No, that’s not the effect. If you make a website in the US hosted on US servers that does not obviously target german customers then you don’t fall under german jurisdiction. It’s not sufficient that your site is generally accessible from germany.
However, if you do add things as “we ship to germany” to a shop then you fall under german jurisdiction - which is fine in my books, because the customer should have a simpler legal recourse than the company.
Having to conform to foreign laws is just the flip side of being able to reach those people easily.
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.
It's false though. The court says German law applies because German users can access the site. German language is only briefly mentioned, but not the argument of the court.
So no, not an extraordinary or new theory of international law.