Most professional photographers rarely use IS or even turn it on.
Agreed. I am technically a professional (reporter/photog with a weekly newspaper) and I have IS turned off on the lens that has it (24-105) and I did not bother buying the IS version of the 70-200 f/2.8 because at the speeds I shoot, it's not going to have an effect.
Apart from many other things, I spend an awful lot of time with reporters etc (and have done for a couple of decades) and I cannot remember a single time noticing one of them had any of their cameras with IS off. Why would they? Why would you? Makes no sense.
There could be some weird feedback loop with high frequency vibrations in some cases.
That could be a reason to turn IS of.
But I don´t get why he has not used a faster prime lens ?