You two are correct. If the shover had continued to advance in a threatening manner, I could see SYG. But he didn't. Violence was over.
Some type of manslaughter at the very least. I don't even see how SYG could be twisted to cover a retreating perpetrator who only shoved and didn't present a fist, much less a weapon.
I tend to agree with you, but we do not have the benefit of knowing what Dindu was saying to shooter. If it was something like, "I'm going to kill you, MFer" that would mitigate in favor of justification.