In the Constitution
Article 1 Section 2 lists requirements to be a Representative. “...and been seven years a citizen of the United States...”
It lists the requirements to be a senator as “...and been nine years a citizen of the Unites States...”
Article II, Section 1 lists requirements to be president — “..no person except a natural born citizen...shall be eligible to the office of president...and been fourteen years a resident within the United States.”
It is obvious that a “citizen” is not necessarily a natural born citizen, and the Naturalization Act of 1790, passed by Congress, directly defines natural born as birth from citizen parents.
Harris is not natural born.
It states a legal fiction. It isn't a definition, unless "legal fiction" and "definition" are taken as synonyms.
Couple modern examples. A patent application is considered as received at the USPTO the day it is deposited as US Express mail. It is actually received at the USPTO a day or two later, but it is considered received when it actually hasn't been received. Legal fiction.
A person 26 years old is considered as a child under certain tax statutes. A person is considered a legal adult at age 18 most otherwise. The tax child is a legal fiction.
The 1790 naturalization act uses the "shall be considered as" language. This is not the same as "is."
Some legal fictions are theoretically out of constitution bounds. A 30 year old shall be considered as 35 years old for purposes of eligibility to POTUS, for example, contradicts the constitution. But otherwise, re-aging is a legit legal fiction if Congress feels like it.