This used to be true but was adjusted with a correction to the internation postage rates. These days things like AliExpress have stopped using local postal delivery and instead use their own local logistics partners for last mile delivery, and it's surprisingly still just about as cheap.
I'm guessing these last mile deliveries are gig workers, considering I seem to get deliveries from people in unmarked personal vehicles nowadays.
The US is already far behind the economies of scale of Chinese production, and has higher labor costs and higher regulatory costs (both good and bad), so it's fundamentally not possible to compete on price alone, regardless of any postal treaty issues, which, again, are not a huge factor these days.
The new logistics services have been pretty interesting, and don't bode well for UPS/FedEx who have been content to focus only on large packages and charge ridiculous fees for a long time without innovating.