They made up their own definition to apply to the case at hand. The case has absolutely nothing to do with natural born citizen requirement to become president of the United States. Was the child a citizen, yes, that is what they ruled. Was the child running for president? No, this case had nothing to do with that.
Such is the import of any SCOTUS decision. The Supreme Court only rules on the "case at hand", right?
The principle of judicial review was established over 200 years ago with a case about judicial appointments. But, that principle has been applied to every case since - almost all of which have NOTHING to do with judicial appointments.
This is why the Supremes are so judicious with the granting of cert, because despite the merits of the individual case before them, whatever they hold as a matter of law, will be applied to all subsequent cases. So, they cherry-pick the cases that best allow them to establish precedent. Thousands of appellants petition for certiorari, but about 100 (on average) are granted cert. Why is that?
If you ever have time, you should search for the number of cases where Roe v. Wade is cited, that have absolutely NOTHING to do with abortion rights. You might be surprised how frequently Roe - and its principle of "right to privacy" comes up.