The world isn't divided into the countries that are under American hegemony and the countries that are under Chinese hegemony. It's more that there are the countries under American hegemony, the countries on mostly-friendly terms with the Americans but able to say no to them, and the small number of countries that are entirely cut off by the USA.
This last group tends to not want to just trade one hegemon for another, they try their best to become self-sufficient and able to say no to everyone. Iran is very much in that category: just because they're on friendlier terms with China than with the USA doesn't make them beholden to them.