For the record you're linking right wing think tank bullshyt right now. Including a legal argument from John Easton, who is currently disbarred due to his disgraceful involvement in Trump's attempted 2020 coup.
His mother was not eligible to become an American citizen and never naturalized. Furthermore the SC ruling makes it pretty clear the case was about birthright citizenship:
"The Fourteenth Amendment affirms the ancient and fundamental rule of citizenship by birth within the territory, in the allegiance and under the protection of the country, including all children here born of resident aliens, with the exceptions or qualifications (as old as the rule itself) of children of foreign sovereigns or their ministers, or born on foreign public ships, or of enemies within and during a hostile occupation of part of our territory, and with the single additional exception of children of members of the Indian tribes owing direct allegiance to their several tribes. The Amendment, in clear words and in manifest intent, includes the children born, within the territory of the United States, of all other persons, of whatever race or color, domiciled within the United States. Every citizen or subject of another country, while domiciled here, is within the allegiance and the protection, and consequently subject to the jurisdiction, of the United States. His allegiance to the United States is direct and immediate, and, although but local and temporary, continuing only so long as he remains within our territory, is yet, in the words of Lord Coke in Calvin's Case, 7 Rep. 6a, "strong enough to make a natural subject, for if he hath issue here, that issue is a natural-born subject;" and his child, as said by Mr. Binney in his essay before quoted, "if born in the country, is as much a citizen as the natural-born child of a citizen, and by operation of the same principle."