LEGAL residents. Wong Kim Ark specifically noted that the alien parents HAD to be present "in amity" with the US government:
"...but were predicable of aliens in amity so long as they were within the kingdom. Children, born in England, of such aliens were therefore natural-born subjects. But the children, born within the realm, of foreign ambassadors, or the children of alien enemies, born during and within their hostile occupation of part of the King's dominions, were not natural-born subjects because not born within the allegiance, the obedience, or the power, or, as would be said at this day, within the jurisdiction, of the King."
Wng Kim Ark strongly suggests the children of ILLEGAL aliens are NOT citizens at all, being disqualified as both NBC and as not falling under the 14th, since the 14th "...to exclude, by the fewest and fittest words..." those already excluded under NBC.
Note that prior to 1882, there was no such thing as illegal immigration to the USA. Prior to that, anyone could enter at any time legally.
The author seems to argue that federal legislation would be required to clarify the question with finality. It could be that one more direct opinion from the Supreme Court is all that is needed, which could come from an executive order and the resultant challenge. Oral arguments by the Solicitor General could refer simply back to that same Ark case.
Or Congress might hop in and attempt to clarify. But that seems doubtful due to such close margins in House and Senate. And they’re wimps anyway.