What if this other mother posted on here:
"I'm autistic, and my daughter was invited to a party and I wanted to make sure to respond to the invitation as I knew it would be considered rude to ignore it. So I saw the mother in the playground and explained my daughter wouldn't be coming, and why. Now I've heard she is telling everyone I'm a bitch."
And if so, this sort of thing probably happens to her every day; people misjudging her motives, labelling her as rude, insulting her, and rejecting her.
If you don't know anything about her motives, why not give her the benefit of the doubt, and take it as neutrally informative, or intended to be helpful (e.g. so you didn't waste effort inviting her again in future, etc), which may be how it was meant?
It's OK for children (or anyone else) not to like each other, and it shouldn't inherently be an insult (and taboo) to say so. Surely it's much more immature to lie about it so you can avoid any potentially uncomfortable consequences of being honest?
I haven't done this exact thing, but I once invited a child (via their parent, so they didn't know about it themselves yet) to my daughter's party without consulting her (she was 6, so was choosing who to invite herself for the first time), and when she told me she didn't want him to attend, I apologised to the other parent and told them that. I wouldn't have lied about why they were uninvited. But maybe she was offended and called me a bitch to her friends.