That I do not know. I suspect that the US asked El Sal to hold him in prison (and paid for that service). Maybe the point is to "encourage" illegal aliens to self-deport before ICE grabs them and deports them whether it is convenient or not. Better to leave un your own timeframe. (settle debts, close accounts, find somewhere else to go). If you wait until ICE deports you, you may go to CECOT, which looks like no fun.I get Garcia had due process and should be deported. But why is he in prison? Deported illegal immigrants are not just returned to their emigrating country?
Welker touched on this a bit. The president of El Sal said he could not send Garcia back to the US. Trump said he has not asked. I think if they wanted to, it could happen in a day.
If it were me, I'd fly him to Brownsville, have a judge and all the witnesses needed on hand, bring him him before the judge, decide his case, put him back on the plane and fly him back to El Sal.
As for the deportation, it could well be that the Immigration Judge amended the withholding order to reflect he cannot be deported to either El Salvador or Guatemala. If so, Trump Administration is in some trouble, but I would like to see the amended order.