USCIS forms aren't the law. The statute is the law. And however USCIS has chosen to structure its forms, the legal basis for a K1 visa holder's eligibility for permanent residency isn't the K1 visa itself, but instead other provisions of the law that confer eligibility for permanent residency on immediate family members of citizens.

This CRS report examples: https://www.everycrsreport.com/files/20151215_R44310_34af119...

"Once in the United States, the K-1 nonimmigrant is required to marry the U.S. citizen petitioner within 90 days. K-1 visa holders are permitted to work in the United States during this time if they file for employment authorization. The foreign national is eligible for lawful permanent residence as an immediate relative if the marriage takes place within 90 days and the fiancé(e) is otherwise admissible."

It's the status as an immediate relative of a citizen that confers eligibility, not the visa itself.