Lessons learned all around. Your daughter should learn that younger children often look up to older kids, and mistake her apparent "charity" in allowing a 6 year old to join in with older kids' behavior as "friendship", which you say it is not. Although you say the kids are not into the same things, clearly they spend time lots of time together (as kids in neighborhoods have done for generations, rather than segregate entirely by age).
Your daughter, and the 8 & 9 year old girls, have learned that you don't talk about invitations and events with people who were not included, because feelings get hurt.
The 6 year old is learning - the hard way - that not everyone gets invited to everything. Unfortunately, she has not been comforted by her parents, who have chosen to take up her cause by being snarky with you. So that's a good example for you and your daughter to discuss - she's going to get left out (or "not be invited", more accurately) in the future, and she can think now about how she's going to avoid getting her nose out of joint. And you're going to be tempted to get involved in some situation where she gets her feelings hurt - and this episode will remind you not to do so.
I do think you might have foreseen some of this - the 8 and 9 year olds likely felt honored or a little puffed up by being invited to a party that was almost entirely made up of the older Girl Scout troop, and perhaps they talked about it in the neighborhood. So in retrospect, you might have kept the party just to the troop, or discussed with the other 2 girls that we don't talk about these things among those who were excluded. Not that the 6 year old is your responsibility, but who needs this drama.
I think you let it go. I doubt that explaining anything to these parents would do much good except keep the rancor alive. Either it blows over and the kids can go back to playing together (perhaps not as often as before), or it remains an open wound and the parents find other 6 year olds for their daughter to play with.