NAL either but D. J. Bernstein [https://cr.yp.to/publicdomain.html] says that you can use a work under public domain in Germany same as any other country (e.g. copying), but there's a separate right to be credited for the work where applicable and protection for the author's reputation.

So if I made Benchy longer and turned it into a rendition of the Titanic, I can't claim that it's 100% mine and no one else had a hand in making it under those rules.