into almost negative numbers
Not almost, negative. When a voice coil is flying backward (relative to the now-changed drive signal), it is a generator of back emf.
We can argue about how long it is out of phase, but the fact remains.
Just agreeing and reinforcing what you said. You were too timid :-)
Now, the practical question is "how well does the amp control the voice coil (aka damping)?" If its very, very good, this overshoot and negative emf will be less. Not zero, but less. No i have not measured - its very hard to do so with non-repetitive signals which is where the problems would likely lie.
BTW i found the blog interesting but need to go backa nd think about what the (other party’s) original chart really meant. Unloaded? Hard to say what it is. Amps frankly are pretty easy if it weren’t for the damned loads :-)