Wiki cites the DVD commentary as the source of this. I've not listened to the commentary in so long - I don't remember.
Perhaps it's 100% true, but it sure would've been weird to get a clearly too old for the role Newman as Superman.
Jor-El would've been most logical for him. Luthor would've...
TBH, I don't think Brando was great in "Superman" or "Apocalypse Now".
Granted, I find it hard to objectively judge his performance in "Now" because I dislike the final 1/3 of the movie period. Even the greatest performance ever wouldn't save that segment for me.
But "Superman"? He's...