It's been a while, but to the best of my recollection, the rules were "Stand up only, no kicking, no ground fighting." It was basically very poor and vague rules. Gannon heard no kicks, so he thought knees were okay (A big stretch) and stand up only, so why not standing guillotine, especially when the guy shoots on you.

All that matters is that Gannon took everything Kimbo could throw back then and still dealt out more than Kimbo could handle. That being said, Kimbo is FAR from the same fighter he was back then.