You were absolute in your response and went further calling out any postern who would even "theorize" that McD may not be able to do both.
You base this opinion on others doing it. Other HCs have won multiple SBs--but it's certainly uncommon. Likewise, few HC's are also the listed OC/DC.
"They exist" isn't a great way to convince someone that McD is absolutely, without question, one of them....as you state is your opinion.