Slightly separate issues. We didn't interdict Iranian tankers at sea until very recently, despite the sanctions. But we are doing it now, of course.

That said, China has a long record of doing business with anyone and avoiding ideological conflict on all issues except for specifically Taiwan. They did business with Pinochet's Argentina in the 80s, while Argentina was throwing socialists out of helicopters and China was still fully communist. Unless you're a pure American forpol shill, there's no reason to suspect they'd restrict free passage in their area.