If you ever get mugged, keep it simple. O-soto-gari can be done on a shirtless guy.
Edit:
I'm not an expert in grappling, though. The extent of my knowledge comes from traditional karate grappling and a couple of years in Japanese jujitsu.
I think ts is discussing the usage of of the GI and it effectiveness for self defense.
Let say isoto gari is the technique. But you only trained, learned and sparred using this technique with GI grips.
Suddenly when attacked in the street, you could find it hard to use it because you are over relying in the gi grips.