No, there was already a mechanism in place to end processes when you log out - when the controlling tty disappears, every process in the session is sent a SIGHU

No, there was already a mechanism in place to end processes when you log out - when the controlling tty disappears, every process in the session is sent a SIGHUP and SIGCONT signal. The processes started under screen(1) are deliberately created with a new pty as controlling tty and in a new session, because they are intended to survive the ending of the original session where screen was started.