The wka and wako who seem to be the main orgs in American style kickboxing both still have a minimum kick rule.
Bearing in mind that the skill level in the style was pretty low and that kicks above waist doesn't allow for much inside game beyond punching, that yes, a world class boxer could have stood a good chance of a ko.
You can't win with low kicks if the boxer has his legs conditioned. The best thai guys are half the size of western boxers. You think they would get the better of them in a clinch?