We contacted our lawyer and he said that the reasons mentioned in the letter are rubbish. Our blog-article doesn't concern any trademark-violations and it certainly has nothing to do with GDPR.
The only critical point might be, if we said something wrong about them. But we have the facts and there are lots of other sites that say the same thing. So we won't delete or edit our article and we'll see if they really go to court and risk losing.
I hope some of you will do the same - that will make it easier for all of us!