The question of whether Abrego Garcia is a member of MS-13 is irrelevant. He was deported under the Alien Enemies Act. That law requires
"That whenever there shall be a declared war between the United States and any foreign nation or government, or any invasion or predatory incursion shall be perpetrated, attempted, or threatened against the territory of the United States, by any foreign nation or government"
We are not at war with Venezuela. An "invasion" has to be a literal invasion, not a metaphorical invasion. Note that 1798, it might take months to summon congress and declare war, which is why authority was granted in case of invasion. Also note that "declared war" is specified. At the time of enactment, the US was engaged in naval operations against French privateers, which was not seen as a justification for deporting every French citizen in the US.
Abrego Garcia may deportable under other grounds but not under the Alien Enemies Act which is what was used. Therefore, his deportation is illegal.