1. It's not necessary, for instance, that the person being referred to by the kid is "known". It's quite common, for example, for kids to address an unknown stranger on the street using "uncle" or "aunty" - it's merely a respectful way of addressing an older person.
2. When used by an adult, it can take on one of two tones:
a) The genuinely respectful tone, similar to (1)
b) A derogatory way to taunt a slightly older person on account of their age, using outdated practices, etc.
The way it's used in THIS article, borders on 2(b) - taunting an older generation for sending good morning messages, etc.
I hadn't personally encountered the 2(b) form you reference. When I was a kid, even an implied taunting of the older generation was forbidden, treated as stepping out of the bounds of social protocol, and would get you punished by your own parents or family if observed.
Even now, as an adult, I would feel strongly compelled to stop myself from using the 2(b) variant you just referenced. It feels taboo.
Maybe things have evolved a bit since I left India, which I admit was quite a while back. I wonder if my grandmother might lament this change as a deterioration of values in the younger generations :) "The kids have no respect these days".
(in addition to the other usages msravi noted)
There are other words used like that, eg “mate”, which can be used in a friendly/neutral or menacing way.
(As a side note - I notice that Kamala Harris’s Twitter Bio currently includes “Auntie”.)
There's such thing in Russian too. My parents had friends and they would come to our apartment every once in a while. We would call them all "uncle $name"
A child will simply not be introduced in a familiar way to someone you don't trust or respect - they will be kept away. If the person is referred to for some reason, they'll be referred to as "that person" or somesuch.
When I ask my son to refer to someone as uncle or auntie, it means I'm signalling to him that they have some degree of standing in my life: they are a colleague, or friend, or acquaintance to whom I am asking him to show respect on my behalf.
An uncle or auntie is someone who "is known". By bestowing that title, and instructing your child to use it with them, you are granting them status in the eyes of your child, and segregating them explicitly from the general set of all older people.
When the term is used by adults, it naturally refers to the _even older_ generation of the elderly. So in this case, where the perspective of the writer is an adult perspective, it refers to the elderly.
Lastly, there's a subtle detail that should be made explicit. When it's used "in general" like it is in this title, it does not imply that I, or any reader, considers all of those people uncles and aunties personally. It's implicit in the usage that we are referring to "people who are uncle and aunties for those whom they are the uncles and aunties of".
I hope I've done a half-decent job, because the entire set of cultural associations around that word are hard to get across.