It makes perfect scene. Renly wasn't interested in the throne for Ned so he would have left regardless and Sir Barristan is a kings guard who has no place to comment. So even if he did say in person, he would still only be talking to Ned because everyone else in the room was already in on Cersei's plot (Pycell, Joffrey etc) . And if he told Ned in front of Cersei, she would have simply denied it and said that he had no proof. That's why Robert gave him the king's seal to give him some sort of proof.
Literally nothing would have changed if he said it to everyone while he was alive because even then there would be no one to support Ned but Sir Barristan, who's a king's guard anyway and can't do anything. Cersei would have gone through with her plan because she doesn't care what he said or gave to Ned or to anyone.