You are correct, something could be done, but it would require amending the constitution. Maybe that could be done, but not anytime soon (and I still question whether it would get the support of 38 states).
But Huck thinks he can amend the constitution on marriage and abortion. My point is people are more upset about illegal immigration and if he can’t get that changed he sure is not going to get the other two changed, but his supporters are sure he can.
I have not heard him say, but it would not surprise me if he did not support amending the constitution to change citizenship rules. This would be consistent with his belief that you do not punish the children for the sins of the parents.