> Ie, China could sell to other countries that are not as concerned about dumping. ... For example, why would a poor country without a local smelter even care about "dumping" - it would seem the cheap prices would be a subsidy in effect.
Because you can either be part of the group of powerful nations that rules the planet (including through various global and regional trade arrangements, international organizations, military power, sanctions, banking/finance and so on), or you can be on the outs with them and they may choose to brutalize you at some point in that case.
The US, as one example, has a lot of levers. If you decide to try to operate outside of its preferences, it can trivially wreck you if you're a small nation. China for its part increasingly swings a big influence hammer, which is why so many nations are very afraid to even offend them. If you're a stray midling country out there, you generally don't get to just do whatever you want to, even if you're not signed on to various limiting agreements around eg dumping. The more powerful nations have specific interests, a specific way they think the world should be, and they have no qualms about being randomly hypocritical (at your expense).
Even most larger nations have to be quite careful. The West is growing quite tired of Erdogan for example and they will further crash Turkey's economy (further amplify the damage Erdogan is already doing) if they think it'll help get rid of him. Turkey can rather easily be smashed at this juncture, potentially resulting in civil war, rolling instability for years or decades, quasi Syriaification.