I don’t much like the year old Guardian article either. The title suggests that President Obama could just do it, but the text only refers the reader to the opinion of former officials–hardly persuasive.
Even if President Obama had used his full authority to close the Guantanamo facility, it would not have been the end of warrant-less arrests and indefinite detention. It would simply have meant having prisoners indefinitely detained elsewhere.
When President George W. Bush closed the Abu Ghraib torture facility, it did not mean the end of torture. It just mean the transfer of torture to another location. The same would have been true of the Guantanamo closing. It would have been a symbolic action, but it would not have freed a single prisoner.
Everyone held in custody by the U.S. government should be treated as an accused criminal, with the right to a fair trial under the Constitution, or as a prisoner of war, with the right to humane treatment as guaranteed by the Geneva Convention under international law.
Sadly there are many precedents, under both Bush and Obama, if President Trump desires to ignore both.