You're reading an assertion into those words that isn't there.
The Justice said that the child of the resident alien is a citizen, not a natural born citizen.
This is a very common misreading of this statement, which I've seen on FR many times.
“The Justice said that the child of the resident alien is a citizen, not a natural born citizen.”
Naturalized are citizens, too. But the decision didn’t say Wong Kim Ark was just as much a citizen as naturalized citizens; it said he was just as much a citizen as natural born citizens. Meaning he was something more than a mere citizen. Apparently everyone who interprets that passage to mean nothing more than “he’s a citizen” forgot about naturalized citizens, which doesn’t surprise me. Because bringing up that other category distracts from the delusion that soil babies and blood babies are polar opposites, whereas in reality both stand together against the naturalized.