If somebody breaks German law, that's their problem, and this is the convention for matters outside of the Internet, too. US CBP, for instance, doesn't hold the seller responsible for complying with import laws and duties, that's the responsibility of the importer. That's because it would be enormously complex to figure out the laws of every place someone is meant to sell to, whereas the buyer only needs to know the laws of where they live.
In the case of Project Gutenberg and Telegram, the German side is the consumer, so it's entirely comparable. If the app or website is illegal in Germany, Germans who use it are the ones breaking the law here.
If I sell chocolates in America, and somebody in Germany where I have zero presence buys them, I should not expect to get arrested if I step foot on German soil because I used an ingredient banned in Germany. If I run a blog in America, and somebody in Germany comments 'Heil Hitler', I should not expect to get arrested if I have a layover flight in Berlin because I didn't delete that comment. To say otherwise is absolutely deranged. Germany can sieze my chocolates in customs - that's fine. They can block my blog - that's also fine. Your implications here are in utter contradiction to existing international law concerning the import and export of goods.
I challenge you to consider how it could be any other way - if the onus is on the other side, why stop at the website operator for not blocking Germans from their site? Is the host not also responsible? The ISP? Should FedEx be responsible for not inspecting their packages for illegal chocolates?
There are plenty of middlemen. There are the hosting services. There are the ISPs. There are the CDNs. If you're going to make Telegram legally liable for these matters, then why not AWS, ISPs, and CDNs?