Webuiltthisbuffet you need to seriously lighten up. They're kids, it's Christmas: time for fun and laughter and joy and kindness. You do you, we'll all do us. This is NOT a thesis about the ills of society.
So are you saying that the 9yo kids were wrong to be discussing between themselves something that clearly interested them, or that the older child telling their younger siblings was wrong for 'not doing them' (their family)?
This is my thing, see: I have no problem with people having fun and family traditions; and I don't really see any great issue with introducing the Santa thing and letting little ones enjoy a make-believe world where reality and fantasy aren't always easily distinguishable.
My beef is when an older child initiates doubt and asks their parents, and those parents firmly push the story, tell the child that it IS true and that they shouldn't question it. The point where 'magical' fantasy becomes an outright lie (even gaslighting) - and then start to frantically (and often angrily) remonstrate with those (often children, even if via their parents) who question or doubt and/or share their thoughts on it.
Little kids believe all kinds of silly things - often family traditions intended for fun and 'magic'; but all of this could so easily be avoided by either telling them the truth or simply saying that not everybody believes the same things - in many areas of lives - and telling them that they, like everybody else, are free to decide what they do or don't want to believe in.
They could equally tell the St Nicholas story (even though the Santa myth arguably originated long before he was born) and mix in the wider 'spirit of Christmas' and generosity and kindness in, without emphatically saying either that 'Santa Claus as a person does not exist' or red-facedly insisting that he does.
Several perfectly reasonable, age-appropriate ways of handling it, if you have chosen, like most, to be a family that 'does' Santa - there's absolutely no need for angry phone calls or messages, bare-faced lying and/or insisting that other people's children must be sworn to silence and not allowed to discuss certain topics that interest and affect them.
As you correctly say, these are people who are emphatically not willing to 'do them' and let other people 'do them'.