Don’t know who Immihelp is but here is there interpretation.(U.S. citizen or citizen of the United States) to include U.S. nationals (but it could).
U.S. National - Difference between U.S. Citizen and U.S. National
IRS interpretation
Alien An individual who is not a U.S. citizen or U.S. national.
U.S. National An individual who owes his sole allegiance to the United States, including all U.S. citizens, and including some individuals who are not U.S. citizens. For tax purposes the term "U.S. national" refers to individuals who were born in American Samoa or were born in the Commonwealth of the Northern Mariana Islands who have made the election to be treated as U.S. nationals and not as U.S. citizens.
Immigrant An alien who has been granted the right by the USCIS to reside permanently in the United States and to work without restrictions in the United States
Nonimmigrant An alien who has been granted the right by the USCIS to reside temporarily in the United States. Each nonimmigrant is admitted into the United States in the nonimmigrant status, which corresponds to the class of visa with which, or purpose for which, he entered the United States.
An individual who owes his sole allegiance to the United States.
Merriam Webster dictionary defines a citizen as “a person owing allegiance to and entitled to the protection of a sovereign state.”
Black’s Law dictionary defines a citizen as “a person who owes allegiance to, and may claim reciprocal protection from, a government.”