Yeah, it can be a bit difficult to try and read between the lines when it comes to narrative postings in online forums (versus being able to benefit from body language and vocal inflection).
I can understand your experience as a firearms instructor when it comes to working with what students have selected and brought to training.
While my primary focus and responsibility has been serving as a LE firearms instructor, I've also taught, or helped teach, a fair number of classes that were made up on non-LE. In those instances the students had already chosen and acquired their weapons. I was just there to work with them and the equipment they possessed, not to make judgments on their choices and equipment.
Of course, I feel pretty much the same way when it comes to working with LE and authorized secondary & off-duty weapons. In those circumstances, as long as it's approved, somebody's choice isn't really my business. It's not
me that's going to have to use it they decide to invoke their peace officer status to take an official action on their own time, or protect themselves or their families. I don't make the rules. I just deal with trying to help them possess the requisite knowledge, training, skills and (hopefully) mindset to enable them to perform as necessary.
Sure, occasionally a student might discover they'd chosen something that was less than ideal for them, and might want to change to something else, but it wasn't at my prompting, or because I denigrated their choice. I was there to teach, not sell.
Of course, there were any number of times I'd wonder privately why some non-LE citizen owner might have chosen a .22 or .25 pistol as their dedicated defensive weapon, but in the long run it wasn't my business (or life that was ultimately going to be at stake, or the lives of my family). Sometimes you could see such folks compare their range performance against that of folks shooting centerfire calibers, or other handguns which were easier to use under stress, but then it would seem they'd somehow shrug off any concerns and express how it would work "good enough" for what they envisioned requiring.
I see no reason to assume that a .223 pistol user interested in SD use would go for the cheapie ball ammo any more than a 9mm pistol user would do the same. Yet I don't hear anyone warning folks away from 9mm pistols in the way that we are hearing warnings against .223s.
And yet it's not hard to encounter a surprising (dismaying?) number of folks who feel 9mm ball isn't really going to be any "less effective" than some hollowpoint. Probably a more common perception among some .45 users ("a .45 never shrinks!"), granted.
Like you, I'd be more concerned with the inherent muzzle blast & flash signature issues for most average owners who tried to employ an AR handgun outside a range, especially within an enclosed environment. I've had a .357 Magnum fired next to my head inside a dwelling (no hearing loss/damage, miraculously), and it was an attention-getter, but a .223 (or especially a 5.56) going off close to me, especially inside a small room or hallway, is NOT something to which I'd wish to subject myself (or my family members).
In another vein, though, how hard is it to find folks who, for some reason, seem to equate muzzle blast/flash with "power" & "effectiveness"?
Those are the folks who might not realize the diminished potential for the type of yaw/destabilization to occur (that's connected with violent fragmentation) when velocity is lost in really short AR barrels. I remember it being an eye opener for some folks many years ago when we were testing some different loads/bullet weights out of an 14.5" AR. The rounds that were ordinarily pretty effective when fired out of longer barrels didn't do so well in the shorter barrel (denim/gel & windshield glass/denim/gel testing). One of the heavier bullets did surprisingly well, though.
In subsequent years, when there was talk of adding some AR models with even shorter barrels, I didn't get involved in further testing, but I remember a couple of the other instructors interested in the subject were considering potential loads and bullet weights that wouldn't be as likely to compromise actual terminal effectiveness when really short barrels were used. We're talking barrels down in the 10.3" length, and with 1/7 twist rates (which creates another concern affecting bullet weights used).
Of course, using such a unique pistol for a critter or varmint gun might suit some folks just fine.
Then again, the Remington XP-100 & XP-100R seemed to have attracted enough interested owners.
Probably not for a dedicated self defense weapon, though.
Best regards.