Trump used the Alien Enemies Act because he declared that Tren De Aragua was Directing the activities of Tren De Aragua and it was an invasion by a hostile nation. This ultimately led to them deporting people to the El Salvadoran Gulag.
Internal memos prove that the US Intelligence Agencies were asked to confirm the links between the gang and the Maduro government but instead they concluded that Maduro was NOT directing the activities of the gang. They informed the Trump admininstation of this more than a month ago, but Trump had already made up his mind so he just went ahead and did it anyways