are you saying it should not have said "As most of you know", but instead "As many of you may know" ?
My 2 cents: while I do not consider the "most of you" such a wildly inappropriate thing to do, I do think that "as many of you may know" would have been OOCly more polite to other players.
First requires that I need to make a decision: 1) accept the suggestion that my character also thinks "most of us" know this, 2) question the posting character's idea of of what "most" know and think him/her a pompous ass for presuming that his/her customs are The Customs, or 3) declare my character belonging to a minority that does not know stuff that "everyone else" knows.
While any of those decisions are pretty easy and valid (and so this is not a
big problem), phrasing it as "as many of you may know" does not require me to more than to decide if my character does or does not know. Whether "most" capsuleers know will be the sum of these decisions made. And so it feels more inviting: I can interact with the festival itself without having to take a stance on how common it is in the world in general.
It's not a choice between "just play whatever CCP has confirmed" and "make stuff up for ourselves". That's a false dichotomy.
You can make stuff up without assuming everyone (or most people) in the universe know about it. In general, I would advice against trying to define "stuff all X know about". It is fun and necessary to write background, but as you will never be able to distribute your background to all players of the race in question, it is much easier and more respectful to define "stuff some X subscribe to" and let people decide for themselves when they first run into it if they want to accomodate it, or invent something else, or keep stuff they invented already to contradict it. That also allows you to play with more people. If you define something as "the X way most if not all X know about", and someone else somewhere else had defined something contradictory as "the X way most if not all X know about", you cannot very easily play together.
For Matari this is easy: we can always do stuff for our clans, rather than a tribe or "Minmatar" in general. But then again, the EVE universe is
huge, and races span whole planets. There's room for local variations for every race.
I am not sure if I explain this very well...