His comment is bogus plain and simple.
He talks about the weight a single player's death has, yet Savage is still a thing. No one is asking for 3 savage tier boss fights for 4 players. Harder and challenging doesn't necessarily mean "Mythic+" or "Savage". And they've done 8p dungeons in the past, which were only ruined by being not balanced for 8 players properly and being loaded with cutscenes.
In fact the only ones imposing the 4-p limit to dungeons are the developers themselves. But oh well this is Yoshida for you. The man who would rather abandon an idea entirely if they execute it poorly instead of refining it, since 1.19 (you don't see hamlet defences or the bonus chest system anywhere do you?), unless it's another version of Diadem.
Moreover theorical optional dungeons need to set themselves apart from trials and normal raids (which are just more trials let's be honest) and even from alliance raids. There should be more to how they're designed that makes them distinct, and the challenge shouldn't all fall on the bosses or even the combat.