Enforcing Congress's laws isn't the only power the President has under the Constitution. He is also commander in chief of the military. Telling the military to round up and detain people is an exercise of that power, and does not violate any separation of powers concerns.

When the people are American citizens on American soil, the action may very well run afoul of other Constitutional provisions, such as due process, but it's not an example of executive usurpation of a power that belongs to Congress.