I don't know where you are, but in America you can request SIM replacements with the same number. These should be only in cases where you lose or damaged the original SIM, but either some staff got a bit lenient or the impostor have forged the necessary documents to prove ownership. Oh, and you can request it on-phone too (plus mailbox interception!)
It seems that in this case, the poster alleges that some staff might be actually involved in this process (which in this case, it's game over).
Like, contact the customer service hotline, and since they're sending the SIM card through the registered address, someone must get it in delivery. Alternatively, just literally ask the customer service to change the address. They should checked these kinds of requests, but considering that currently the T-mobile data leak is on the front page, you shouldn't be really shocked on how lax American security standards (except for their military) are.
It seems that in this case, the poster alleges that some staff might be actually involved in this process (which in this case, it's game over).