Yes, the developer had every right to say retard. 100% correct. And if they said it in front of me, I'd have every right to kick them, hard. Both of us will have paid a price, sure, but hey, freedom of speech wasn't violated.
You're free to call me a retard. I'm free to kick you for doing it. Everybody wins. But your calling me a retard isn't a divinely bestowed right to get away with it without a response just because you had the right to say it. GitHub chose to respond. The author chose to comply with the request. Done and done.
Just to nitpick... Battery is the physical act. Assault is telling someone that you're going to kick them, hard, and giving them the impression that you intend to actually do so. Both of these things are criminal, obviously.
You're not free to kick someone that calls you a retard. That is assault, which is a crime in most places on earth. I thought every adult would know that, but here we are.
You're free to call me a retard. I'm free to kick you for doing it. Everybody wins. But your calling me a retard isn't a divinely bestowed right to get away with it without a response just because you had the right to say it. GitHub chose to respond. The author chose to comply with the request. Done and done.