As I suspect I'm going to say a lot in the next 4 years: The devil is in the details.
Mario Longhi already clarified that he thinks the economy will improve in general, which can help him rehire 10,000 workers. He's expressing support for Trump -- which is a good idea, since apparently criticizing him can trigger a Trump Twitter Tantrum, and several hours of bad press and death threats. There is no specific deal, no specific promises of rehiring.
U.S. Steel clarifies jobs numbers tied to Trump | Pittsburgh Post-Gazette
On one hand, I agree with Longhi that if the big infrastructure plan goes through, there's a good chance steel demand will increase, and US Steel will likely be able to rehire workers. Perhaps 1 to 2 years from now. And of course, it's just smart business to butter up a President who gives every sign of wanting to micro-manage any business that is in the spotlight.
On the other hand, it's a joke. China has spent years dumping cheap steel on the entire planet, and was already whacked by Obama with a stunning 250% - 500% tariff, and the EU recently followed suit. Guess what? It didn't help. China has continued dumping, retaliated with its own tariffs, and routed steel through Vietnam to avoid the tariff.
Supply isn't the only issue. Global demand is low, "global" meaning Brazil (massive economic crisis), Russia (getting killed by low oil prices), Europe (which is killing Tata Steel) and... wait for it... China.
Best of all, our very own President Elect shows why the "trade war" approach doesn't work. The guy who wants to Make America Great Again chose that cheap Chinese steel for several of his projects, usually purchasing through shell companies to hide the trail. This is pretty much what millions of American consumers do, they complain about the loss of US jobs, but don't want to pay the premium price for an equal quality product just because it was made in the US.