It's actually a made-up name.

They came up with a list of invented names, including Lenin, Bismarck a couple of compounds like SpongeBob and IIRC Shakespeare, and they thought Clausewitz sounded good. It also reminds of Santa Claus (similarity in italics).

Later Meneth found out it was also the name of an obscure general, so he immediately deleted the code of the Ultimate DLC he was working on with the best historians around (later some of it was salvaged by the HIP guys, then reused by Meneth himself for the Holy Shit DLC) to introduce Clausewitz portraits instead.

This also prompted some guys from /r/crusaderkings to find Clausewitz grave in order to create a meme (a picture of Glitterhoof, but posted while standing on his grave), and they found a book inside it. It then got published and there's your discover.