Even if English is the lingua franca of the world, people master it to varying degrees. Also, it seems like 'quitted' was the more common form up until the ~late 1930s [1], so it's not entirely unreasonable to assume that, if this person learned with some vintage material or they read classics, they've seen 'quitted' more often.
[1] https://books.google.com/ngrams/graph?content=had+quitted%2C...