I agree with your last sentence. For what it’s worth, here is part of what Tina wrote in the epilogue of her book:
I noticed some minor discrepancies between what I recorded in the oral histories and what I now remember. Going through my files helped make some things more clear. If anyone wonders which is more accurate, I would lean toward my book rather than the oral history.
Frankly, I think that relating the time of the first shot to just after the end of her filming segment is something that she most likely would have been able to do accurately (because our memories are quite often based on those types of associations). However, I think that remembering exactly how much time might have elapsed between the end of her filming segment and the first shot would be much harder to gauge accurately. Therefore she gives a range of 1 or 2 seconds (if that much, aka: or less) in her book. That would place the first shot somewhere close to the Z133 time.
If that was all the evidence as to when the first shot occurred, I might find your point persuasive, although 2 seconds would put it around z150.
But it isn't. We have Croft who said that the first shot occurred after he had rewound his camera to take another photo after taking his photo at z161. Betzner said that he took his z186 just before the first shot was heard. Mary Woodward said that JFK's last smile and wave began before the first shot. Witnesses between the lamp post and the Thornton sign said that JFK had just passed by where they were then the first shot sounded. Occupants of the VP car said that their car had just completed the turn when the first shot sounded - it is still turning when last seen in z180. Occupants of the VP Security car said the first shot occurred as they were completing the turn (along side the TSBD) - it is about there in z191 when last seen. Jane Berry, the 7th person west of the lamp post near the Thornton sign said that the first shot occurred as the President's car was passing by her.
So, if one tries to fit Tina Towner's recollection to the rest of the evidence, 4 seconds is a better fit than 2.