Some parts are imported but it‘s probably as European as possible at the moment. Details about production can be found here: https://www.nager-it.de/en/maus/fertigung. Unfortunately their shop seem to be only accessible in German.
credit: https://www.reddit.com/r/BuyFromEU/comments/1ngg5r6/fair_and_european_computer_mouse/
Most important role by far. For example their wood wheel is (currently) hand made.
And I think china is much better at the environment thing then you think. I know they are insane when it comes to chemical waste. Idk about germany, but usa (and my country, and others I know of) is far worse when it comes to polution per product.
Nothing of it matters as I will never buy a mouse without 5 buttons. I could tolerate a wire, barely.
Oh, and the best for the environment is to not buy a new mouse unless you need it. Mine is like over 5 years old and I won't get a new one until it breaks.
Anyway this seems like a good thing they want to do. No matter how bs their cost breakdown page is.