Yes, it is coming from the ROM. Well spotted, it looks like you are right. A quick glance here https://computerarcheology.com/Arcade/SpaceInvaders/Code.html#t

Yes, it is coming from the ROM. Well spotted, it looks like you are right. A quick glance here https://computerarcheology.com/Arcade/SpaceInvaders/Code.html#text-character-sprites https://computerarcheology.com/Arcade/SpaceInvaders/Code.htm... and here https://www.fontzip.com/apple-ii-screen-typeface https://www.fontzip.com/apple-ii-screen-typeface Shows a distinct similarity.