I don’t know the details, but in Tibet, sticking out the tongue as a greeting is a thing many centuries old thing. Don’t know much more about it than that.
But Heydrich Harrer wrote about it in Seven years in Tibet and it was shown in the film. It’s one of those weird third world things.
https://www.youtube.com/watch?v=rMmyMA8oI3U
Thank you for pointing out that the world is often quite different from our own limited and biased views.
Enough with these attempts. Always these kinds of responses from open-mindeds and cultural "experts".
No one sticks their tongue out in front of a boy's and tells him to suck it.