Risetime of these current pulses was provided in a previous
post: exponentially rising and falling edges of 8/20
microseconds.

Obviously the ground wire would not be run through a coil.
But due to the high frequency nature of transients, that
ground wire must not be spliced, no sharp bends, and not
routed inside metallic materials. A violation would only
increase wire impedance unacceptably. Why? Because even
sharp bends cause significant impedance increases when
discussing the higher frequency components of lightning.

We could play a game of 50,000 questions since even the most
basic nature of lightning - its RF components - is new
information. These and other questions have been answered
previously in a list of citations at:
http://makeashorterlink.com/?X61C23DCA

Figure about one days worth of reading.

CJT wrote:
A small multiple of 40-50 Hz is not exactly RF. So what's the
risetime of these pulses? Duration?
...

Why would you want one to run through a coil, anyway?

I suspect the warning is to avoid somebody being electrocuted by induced
current from the pulse(s).