The notion that the U.S. Constitution only protects U.S. citizens is palpably false. It is an indictment of our education system that any American could think such an outrageous thing.The drafters of the Constitution and the Bill of Rights were educated men. They chose their words carefully. They debated over precise word choice. One can assume every word they chose was done with a purpose.
The Constitution and the first ten amendments distinguish between the concept of “people/persons” and the concept of “citizen.” For example, Article I, Section 3, says “No Person shall be a Senator who shall not have attained to the Age of thirty Years, and been nine Years a Citizen of the United States…”.
A “person” is therefore different from a “citizen.” Everyone is a person, but only some are citizens.
Funny thing that those of us who have read the Constitution (and admittedly we’re a small group) can miss stuff like this. It’s nuanced, but that which has been learned cannot be unlearned.
This is why I love hanging around smart people. You always learn something.