The point was that slide velocity rearward isn't dependent on muzzle velocity. Muzzle velocity is a factor, but it's only part of the whole picture.
Well, sorta...
At the precise point that the bullet exits the muzzle, the slide velocity rearward is wholly dependent on muzzle momentum (ejecta velocity x ejecta mass). The ejecta is everything that leaves the muzzle, gases, bullet & unburned powder/fouling. In practice the bullet's momentum is, by far, the dominant factor and for most purposes the rest of the ejecta can be ignored.
After the bullet exits, the slide will slow down based on friction, recoil spring force, cocking effort, etc.
Apparently you didn't read the part where we discussed that "about" 90% of the bullet's velocity is obtained within a half inch of its travel...which means that the slide's velocity isn't wholly determined by muzzle velocity.
The fact that most of the bullet's velocity is attained very early in its travel is absolutely not proof that the slide's velocity isn't wholly determined by muzzle velocity. The slide velocity at the point of the bullet's exit is a function of recoil which is purely based on conservation of momentum.
Therefore, AT THE INSTANT THE BULLET EXITS the momentum of the ejecta is absolutely the only factor that needs to be considered. For practical purposes, the bullet's muzzle momentum (and the mass of the slide & barrel) is all you need to know.
BEFORE the bullet exits, it is necessary to calculate the momentum of the bullet and gases in the barrel to determine what the slide velocity is. However as pointed out, at least in the type of firearms under discussion here, the majority of the bullet's velocity (& momentum) is imparted very early in travel and therefore it's sufficient for practical purposes (you'll get an answer correct to within 10-20%) to use the muzzle momentum for the purpose of determining slide velocity before the bullet exits.
AFTER the bullet exits, then it is necessary to take friction, spring force, etc. into account to determine slide velocity precisely.
Recoil...which is what the slide does...is nothing more than backward acceleration, and its velocity is determined by the rate of acceleration and the force required to achieve that rate.
You can calculate recoil by working with acceleration and forces, but it is much simpler to calculate it by working with momentum. Trying to work with acceleration and forces needlessly complicates the issue in this case and is likely to, as it has here, result in confusion.
By the time the bullet reaches the muzzle, the slide has gotten all the speed it's ever going to with that round.
Absolutely correct, and that's why, at the point the bullet leaves the barrel, there's no need to have spent any time trying to figure out acceleration rates, etc. while the bullet was still in the barrel. Everything you need to know about slide velocity at the point of bullet exit can be calculated from ejecta momentum and the mass of the slide+barrel.
That's the simple version. For a detailed analysis, including a comment on the effects of the recoil spring on slide velocity, the link below should suffice.
http://yarchive.net/gun/pistol/1911_dynamics.html