If someone says "How can illithids exist, they don't make sense" I think it's a reasonable reply to say "You're okay with Flumphs but not Illithids? Really?"
But if someone says "studded leather didn't exist, it's a weird nonsensical fantasy armor," I actually think dismissing this with "yeah but you're okay with fire breathing dragons?" is a categorization error.
It's not as clear a case as say, "yeah, but you're okay with the Spiked Chain?"
But it's still pretty valid. Unrealistic fantasy armor in an unrealistic fantasy game with unrealistic fantasy elements like dragons? Not a big issue.
At the end of the day, all of our fiction has to enable us to suspend disbelief. Everyone has some threshold of believability/credulity that works for them. Those thresholds are gonna be different... wildly different in some cases.
The kicker is when they're wildly different within one case. It's fine to be a purist about leather armor, if you're also at least open to other complaints on the same level - potatoes, for instance, are not uncommonly mentioned in passing, but are anachronistic in a medieval-European setting. The Rapier is anachronistic. There's /plently/ of other errors in D&D on the same level. If you're unaware of some but exercised about studded leather, fine. But if you're selectively indignant/dismissive, well, your complaint may carry a bit less weight.
Make sense?
But overall, I'd say that torches, studded leather, etc. are all items that are generally considered to be part of the "mundane" world of D&D, not the fantastical one.
It's not like there are two separate such worlds. You don't cross a border or step through a circle of mushrooms, or get inducted into Hogwarts, and transition from the mundane to the fantastic - it's a fantastic world. It has some elements in common with the mundane world of historical medieval Europe, and others not. The not range from potatoes, studded leather, and rapiers to dragons, teleportation, and gods walking the earth.
So if someone feels a particular element of "mundane" D&D is nonsensical and breaks their suspension of disbelief, I think that's a legitimate concern.
I believe it breaks their suspension of disbelief, but I don't agree it's a legitimate concern. If your suspension of disbelief is that selectively fragile, either adjust your fragility selector, or get used to doing without disbelief, because you're not putting enough 'willing' into your willing suspension of disbelief.
That said, if a DM wants a world with realistic Brigandine armor and plate-and-mail instead of apocryphal studded and platemail, more power to him. Make the game your own.
I think what he's trying to say is: The existence of the fantastical doesn't mean we remove the onus on the mundane to be within the bounds of credulity. Does that make sense?
It parses OK. It does not seem reasonable.
It's why people change falling damage
The root 'problem' making falling damage unrealistic isn't falling damage, it's hit points. (Sure, it could be a much, /much/ better simulation, but it captures that the greater the height, the more deadly the fall. Which is intuitive enough, however scientifically inaccurate.) Falls from a great height are deadly. We know that. A dagger to the aorta is also pretty deadly, but you don't seem nearly as many people upping dagger damage as fiddling around with falling damage.
Ultimately, heroes in genre survive being stabbed and survive falling from great heights - through various authorial devices (the blade misses the heart by hair's breadth, the hero catches hold of a convenient ledge) - so PCs get hps.
or change non-magical healing, or hate fighter healing, etc. These all stem from the same basic issue,
The fighters-can't-have-nice-things double-standard, yeah. ;P
Seriously, though, that's what pointing out the other anachronistic, fantastic, anachronistic or other genre element is pointing out. That there's a double-standard being imposed.
And, yeah, when it comes to something as utterly subjective as that bar of willingness to suspend disbelief, people /get/ to have double-standards. They just shouldn't be so determined to impose them on others. Let the game have it's less realistic, even on the other extreme, less genre-faithful elements, and just don't use the ones that you have a pet peeve with.