Right, in a nut shell that is it. We don’t deny George was involved with the civil rights movement. We just don’t think the newspapers are lying when they originally reported the two didn’t march together. But they both did march but on different days. Thus Mitt never saw his father march with King and he surely didn’t march with his father and King like he said in a 78 interview.
At issue are the eyewitness accounts of the June 29th NAACP march; two corroborating eyewitnesses. It’s possible both are wrong, but ordinarily that’s considered strong evidence. The 1978 interview has very weak support-only one second-hand source, 30 years after the event. Your gotcha is a few links short of a chain.