(You'll have to read the article to find the context on this.)
If that were true, there would have been people turning their heads toward the sound of the rifle and scurrying for safety (regardless of where it came from) before the next shot was fired. A careful examination of the frames will show no crowd activity. If a shot was fired before Zapruder re-started his camera, there should have been noticeable movement by the crowd until the next shot was fired.
As far as I can tell, I don't see any. But I'm not finished comparing the frames yet.
The dog didn't bark. Nor did the rifle.
There was a motorcade passing through the plaza and a large crowd cheering the President. The rifle shot was not loud enough in those conditions to attract attention.