Absolutely, 100%, no.
A child could be playing out of sight of a parent, maybe a block away with friends, and get mildly injured in a way that requires minor treatment. Or just crying because of a negative interaction with a peer.
This DOES NOT mean children at a certain age and maturity level cannot be trusted to gain some independence and leave their parents line of sight for short but increasingly longer periods of time.
Once he reached 5 and showed they understood the boundaries and requirements, to go out there by himself.
And once he reached 6, he was trusted to look after his 4 year old brother in this space.
And these restrictions seem very strict compared to how I grew up and especially compared to how those older than me grew up. Or in some other places in the world, where e.g. in Japan it's not rare for 6 year olds to ride the subway and do errands and downright common for 7-8 year olds.
Perhaps the parents had clocked-on to this, and were just letting the girl self-soothe so she could learn resillience. Then, on-cue, in steps some member of the public with their own opinion on the child they're trying to raise. This would be kind of tiring for the fatigued parent of a toddler, and the frustration of the parent in the above scenario is justifiable, particularly as encounters like this could happen multiple times daily with a child like that.
Now they could also just be a shitty parent. There's plenty of them. But it's difficult for us to judge and make hard rules in cases like this.
Our society is not as safe as I would like, but it is probably safer than ever before, when children roamed, played, and did errands over wide ranges.
My world was orders of magnitude smaller than my parents'; despite my efforts, my children's world is orders of magnitude smaller than mine. In part, this is because of attitudes like yours, where a child being unwatched is not okay under any circumstance.
When I was a child, me and my friends were gone unattended all day every day.
What a terrible way to live life, always watched over.