Show us where he said there were only two shots. He always said he heard only two shots. He has said many times that he accepts that there were three shots. He has always maintained that JFK reacted to the first shot by hunching forward and to the left.
Re: standing on running board at z306. We do not have a visual record of when Hill stepped off the running board. We know it was after Altgens 6 at z254-255. It is difficult to estimate how long it took for him to get along-side the front of the QM when we first see him in the Nix film. He does not reach the rear hand-hold until z338 which is 4.5 seconds after Altgens 6.
I expect that when he stepped off the running board and hit the road, he was not immediately running at the car speed and probably went back a few feet before running faster than the car. If he left the car at z265 that would give him 4 seconds to make it to the back of the limo at z338.
If he leaped off at z307 that would give him 1.69 seconds to get up to speed and dash the 14 feet from the running board to grab the handhold:

That is not much time. He would need to average 20 feet/second or 14 mph from the time he jumped off the running board until reaching the hand-hold (assuming the limo was moving at 12 feet/sec or 8 mph).
I would suggest that 4 seconds is probably closer to the actual running time.
SSA Hill had jumped offa the running board & was hitting the tarmac when SSA Hickey accidentally fired his first shot of his 4 shot auto burst of his AR15 (this first shot probly wounded Tague).
Later Hill was galloping & almost level with the front wheel when Hickey fired his shot 4 at Z312 (which hit jfk in the head & cracked the windshield).
So, it makes sense that Hill did not hear them there 4 shots.
With hindsight Hill could have turned around & reached over & grabbed the barrel of the AR15 to try to save jfk (instead of jumping off).
But otherwise there was nothing that Hill could have done.
SSA McIntyre who was standing behind Hill was in the prime pozzy to see Hickey shoot (& we see in Bronson that McIntyre was indeed looking at Hickey).
Anyhow, SSA Landis (who was standing on the right hand running board) is now today the only SSA still alive.
And he knows the truth.
They all knew.
https://www.jfkassassinationforum.com/index.php/topic,2836.72.html