Escaping, I obviously don't know these people and clearly them using an inappropriate carseat is a serious problem, HOWEVER, I do have a son with special needs and I got so freaking tired of people treating me like an incompetent mother I eventually just smiled and nodded when people gave me advice about how to get my child to eat/sleep/etc.
Our son had severe reflux and sensory issues and literally did not sleep for more than 45 min at a time for 11 months. I tried everything under the sun, which of course random people who saw me at social events didn't see. All they saw was a very unhappy baby and me "doing nothing" to help him. No I wasn't cuddling him (because he hated it), I wasn't checking his diaper or feeding him (because that wasn't the problem), I wasn't bounding or walking him, because I had found a person who was willing to hold him for 20 min while I had the first adult conversation and break from my poor little sick child in a week or two. I carried him 24/7 and so yeah, when I had the chance to get a few minutes, I took them. He also didn't eat solids until he was over a year which was not because I didn't heat his food. It was because he couldn't swallow. But I had a million people tell me what to cook, how to prepare it, etc, and I did exactly what they did to you which was to say "yeah that doesn't work" or "we don't do that because <silly reason>" just so they would stop telling me what to do.
From the outside I know for a fact it looked "neglectful" because I had numerous people say directly or hint as much. It really sucked and isolated me further because no one seemed to understand. When I said he didn't sleep I swear every single person I said that too either said "yeah I remember it wasn't until DS was 8 months that he slept through the night, before that he woke up two or three times a night" to which I remember thinking "ONLY 2 or 3 times?!" Or they said "You should really try XX, YY, or ZZ" and then assumed I wasn't really trying if he still wouldn't sleep.
Because of COURSE I didn't want to talk to those people about our struggles, considering they weren't any real help, why would I feel obligated to take the time to explain that we'd been to 4 different specialists that week, that we'd tried a million things, that we were literally dying from lack of sleep.
So, I guess my point is that you just really DON'T know and, while you may be right and these people are just jerks, the fact that they shared with you that their child has serious problems and your reactions is "well when I see him he seems fine" is exactly what some of my "friends" did to me. I apologize but this is really a sore spot for me because I felt positively abandoned by people I previously considered friends when they turned into advice dispensing machines rather than actually listening to what I was saying about our struggles and never simply asking "hey you seem to be having some trouble, is there anything I can do to help?"