I haven’t been able to find it since, but during the night of the shooting Fox News showed footage in which the muzzle flash was very visible. They even had a circle drawn around it. Several other posters on here saw the same video.
The footage exists.
I believe you’re thinking of a video of a strobe flashing in a reflection on a window. I’ve watched every bit of the live video, at least once, and never saw muzzle flash.