Yeah じぶん is a really common word that means “oneself.” You use it so that you aren’t redundant with the pronoun you use beforehand.
彼は自分のパソコンを持ってきた。 He brought his own personal computer.
彼は彼のパソコンを持ってきた。<— Sounds strange and misleading.
I don’t know why Koichi said it was supposed to be militaristic, because in anime I have never heard any military officers use 自分. Although they often used 貴様 to their subordinates…
Maybe Koichi meant it would be militaristic if used as a pronoun instead of meaning “self.”