The US Army has never been particular about where a person was born, or whether they are a US citizen. A green card is required, so undocumented immigrants should not be allowed to enlist.
But this isn't about undocumented immigrants. It's about people whose documentation expired while they're on the waiting list for a background check so that they can enlist. UP to now, the 'in queue' status has been sufficient to replace their expired visa, but suddenly they're yanking the rug out from under these people
Many of whom have given up their nationality in their country of origin when they swore fealty to this one. So if they're deported, they don't belong where we'll send them...