In answer to OP- if you are absolutely sure your facts are 100% correct, then yes I would say it is sending a pointed message. From what you've said, I can't think what it is. Again, if what you say is 100% correct, then it would seem that the person you are discussing has been unreasonable to exclude these 3 children for no discernible reason.
In response to the other topic being discussed, I think it is acceptable to exclude a child who is bullying your DC, even if it is a whole class party. I don't think their are many other good reasons though. I do realise it can happen by mistake though, and I think too much can be read into these situations if you put your mind to it. Some people like to be outraged and offended though.
I wouldn't want my child to have to be worried or anxious about the bully being there- even if I could "police" the party to stop it actually happening. Although I would have thought "policing" this is in itself not always easy as some bullying can be quite underhand and not at all obvious, or done where it's not easily picked up. After all, if it's so easy to pick up, why does it so often go undetected by teachers and even by parents? So I don't think I could be 100% sure I could stop it.
It's not fair on the child being bullied. I don't see excluding a bully as bullying- I would advise my child to avoid the bully, not try to become their best friend, unless the bullying was sorted out. And if DD wants a whole class (except bully) party, and I was willing to accommodate a whole class party in principle (I do, but only 10 in DDs class, wouldn't if there were many more), then I wouldn't either make her cut the numbers or invite the bully. I might discuss it with bully's parents, so they knew why, if I knew them/knew how to contact them.
The child who is a bully has to learn that there are consequences to their actions... they should not be included in fun treats at the expense of the child they are bullying, but that's just my opinion.