What the heck does that mean? The whole point is "properly driven." That doesn't necessarily mean super-low output impedance in every case. If that were so, neither OTL or transformer-coupled tube amps would ever enter the picture with audiophiles. Yet they do ... and literally prosper.
Technically speaking, it's possible to over-damp. Every oscillating system will have a critical damping factor. By definition of a critical point, that means there are scenarios on both sides that are not at the critical point. IOW, there are under-damped and over-damped scenarios.
The entire race for a high damping factor pre-supposes the assumption that every single headphone driver will react the same to ever-increasing damping factors. It may work in a lot of cases, perhaps even the majority. Yet, it's also a bit like saying every headphone ever made is the same.
I personally never found the output impedance to matter near as much as people make it out to be. I found as long as the amp gives them satisfactory power and has good synergy with the phone then that's all that really matters with amplification.