That muzzle flash was obviously added, and it had to be because it occurs too soon.
There is another film which you can see here:
It gives us a good view, but there is no visible muzzle flash.
Oswald is starting to cringe there, and then he cringes more- from being shot.
Now Oswald is grimacing and looking pained. So, he was definitely shot. And surely, we would have seen the muzzle flash were there one.
So, the thing Robin Unger put up is just more film fakery, and who knows? Maybe it was done recently. I haven't seen that before, and if it was widely seen, then others would have known about it and referred to it. It's probably 21st century film alteration for the JFK assassination. They are never done. They are never finished. They will fake films, shirts, whatever it takes to keep the lie going that Oswald killed Kennedy.