I would also add that Kafka did very much publish stuff when he was alive; he wasn't a tortured genius writing in secret, he was like, writing short stories for the newspaper. He published a novella of his weird random ramblings on things. He was probably kinda known as the bug guy before he died.

Also like, some of his longer posthumously published books are very obviously not finished. I'd wager Kafka's statement is less a tortured artist thing and more of a "this book straight up doesn't have an ending why would you publish it" or "please don't my editor is probably going to try and finish it" thing. Should you publish an author's clearly unfinished work is a completely different question with different arguments