Yeah there is no such thing as selling a commodity below its market price on the open market. Someone else will come and arbitrage that away, or it’s not an open market.
But you are right someone will arbitrage the price, like scalpers selling concert tickets.
A well-known anticompetitive tactic is for a big player with a vast bank account to move into an area and eat losses while waiting for their competitors to starve. Well observed in the 90's movie rental market, so certainly real.
(Legitimately asking: I don't know the answer ... but based on most US business law my strong suspicion is ... no, they don't.)
By that account China would be bankrupt by now because subsidizing a "buttload" for "decades" does cost a lot of money.
I think the reality is much more subtle - they force their way into a new market if needed and then stop and let the market forces do their thing.
America can't do that because the government takes bribes from banks and believes in Friedmanite ideology where state investment is evil. It's a great opportunity for Europe to copy China and become independent.
To successfully perform dumping, you have to have access to excess supply capable of disrupting that market.
So if that's the expected outcome, it might be better to prevent competitors from building factories in the first place.
A similar thing happened in the railway world, where two state owned railway companies colluded to have used rail cars scrapped so that their privately owned competitors can't buy them: https://www.railway-technology.com/news/ec-cd-obb-antitrust-...