The decreasing volume of the building from the collapse itself could not create enough pressure to cause such localized high-velocity effects, and this early in the collapse would have only created a modest overpressure".
The suggestion here is that the squibs are the sign of a “high-velocity effect”, not the “modest overpressure” that you’d get from a window breaking. We presume the author is saying these squibs throw out an exceptional amount of material and smoke. True? We don’t think so. Consider these images.
The image above shows our squibs and the broken windows (front right of the building). The claim here is that the “squibs” are high velocity events, and that means they should be throwing out more smoke and other material.
Step on in the collapse and you might notice the broken windows are looking slightly more blurry, because the smoke coming through them is stretching out. The squibs, though? Nothing. There’s no significant smoke drifting up, no material falling down, the most you can say is they’re fading away a little.
The broken windows at the front of the building are now not clearly defined at all, and it’s obvious that smoke coming out of them is drifting upwards (or rather, the building is falling). The squibs, though, show no signs of change. The largest remains about the same size, and in the same position relative to the building.
As the building disappears, we see the black smoke that’s emerged from the broken windows appear over its right-hand edge. The squibs, though? Nothing, no sign of ejected smoke at all.
Now, maybe there are explanations for this. There was more damage to that side of the building, for instance, so perhaps there was less smoke to be ejected. Or maybe it was white, not the darker smoke we see here, so not clearly visible (although the “squibs” themselves appear dark).
Of course without any evidence, these are just guesses. The facts, from what we see in the film, are that the “squibs” (or whatever they are) appear no more dramatic (and arguably a little less) than the broken windows at the front of the building. There is nothing at all to indicate they are the results of particularly “high velocity effects”, and no reason to find them any more significant than the other events we can see happening as the building falls.