I give a regular lift to a child my daughter is only vaguely friendly with. Both nice girls - not a lot in common. I wouldn't force my daughter to invite her to a party.
My daughter is 8. At this age, several times I've been told by the parent that she isn't on the party list. Always with an apologetic grimace about the knock on awkwardness of letting them do their own invitations!
I am totally sympathetic, and thank them for telling me.
You said that people were surprised you weren't invited - that is why the mother told you. To avoid you wondering whether the invitation got lost, of if there was a snub. Keeping silent is more of a snub than simply telling you!
It sounds like there is a LOT of backstory. You say there isn't but the thread is littered with you saying there's more to it, she's done stuff before, managing friendships...
You know the backstory, so you know if she's BU to manage the invitations.
But even if she has, the act of telling you that your daughter isn't invited is not in itself unreasonable. And that's why you sound U and a bit dramatic to me! I'm picturing a bit of talk-to-the-hand action alongside your "I don't need to hear it".
It's totally normal in my circle to tip off other mums when their won't get an invitation. No point in keeping quiet. For a start - if your daughter really is such good friends with this child, you'll know when her birthday is.