"America is not Germany. They have their own constitution. I haven't read it. Are all "persons" guaranteed due process there?"
Due process is I am not a citizen and have no constitutional right to be here.
It's the same in the US. Any "due process" is federal policy that exists outside of the constitution. And policy can be changed.
"Like the Fourteenth Amendment, the Fifth Amendment includes a due process clause stating that no person shall "be deprived of life, liberty, or property, without due process of law"."
And that applies to every human on earth? That applied to the Japanese soldiers who invaded the US in WWII? Or the British soldiers who invaded 150 years prior to that?
"I'm pretty sure that after this man is brought back and gets his "due process" he will be immediately deported again."
Why would he be brought back? He is a citizen of El Salvador. He's in El Salvador. He's not a legal resident. If he did come back, it would be an in-extremis exemption to policy-driven almost solely by emotion.
The dude was an illegal immigrant. So far as I know, there is no policy exemption that provides legal resident status because they stayed in the US and didn't cause trouble.
I think this dude (and all the others) will stay in prison until the US stops paying for El Salvador to keep them there. Then they will be encouraged to sneak back into the US.