I gather you're educated but I think you're unfamiliar with a few basic legal tenants here in the United States.
The first is habeas corpus. This is a fundamental legal right and court order that protects individuals from unlawful imprisonment, requiring the government to present a prisoner and justify the detention before a judge. This right of Garcia was violated by DHS. If you don't like that Garcia is afforded the writ of habeas corpus, then you similarly have no objections to you, personally, being arrested and imprisoned in the future without any cause, right to a hearing or legal process.
The second is due process. This is a course of formal proceedings (as judicial proceedings) carried out regularly, fairly, and in accordance with established rules and principles. Garcia's right to due process was violated in multiple ways including his deportation to El Salvador despite a court order prohibiting this, illegal detention, vindictive prosecution, and now coercion.
The third is the fifth amendment. The Fifth Amendment guarantees due process for all persons in the U.S., including undocumented immigrants. Your suggestion that Garcia has no rights to due process, therefore, is simply wrong.
Look, I understand that you *think* that guys like Garcia are guilty until proven innocent and should be instantaneously deported, but that's not how the legal system works. Trump and his goons have played loose and fast with the law to fulfill his campaign promises, but the judicial branch is catching up with him and holding that many of his actions have violated the law. Not only with Garcia, but also with all the folks Trump illegally deported to El Salvador and Venezuela.
Again, you may not like it, but persons residing here in the US have basic legal rights... and for good reason.
@lawgator1 is far more versed than I am on legal subjects, and may have more to add or opine.