Not really. I've played thousands of hours and didn't guess Brandenburg. Took the quiz too literally, or maybe I don't have the right expansion pack, but a few of these are not the flag from the actual game.
The only major differences here are flag shape. There are a few very minor differences but that Brandenburg flag is extremely close to the Brandenburg flag in the game.
Source on Brandenburg: https://eu4.paradoxwikis.com/Brandenburg
Unfortunately, I can't use the actual in-game images for the flags because most of them are copyrighted by Paradox, but I've changed some of them to be more similar to the flags in EU4.
Source on Brandenburg: https://eu4.paradoxwikis.com/Brandenburg