Well, it doesn't matter. If the SCOTUS decides that some people, in certain circumstances, are not in jurisdiction of US law, then they have to apply that notion everywhere.
They can't pick and choose "oh no they are in jurisdiction of law A but not in law B". Jurisdiction is a fundamental concept, there's no middle ground.
As for whether people are really doing birth tourism: sure, there might be some cases, but well, they are using something that the legal system allows. If the country feels like it doesn't want that happening, it needs to amend the Constitution.
(Also, let's not kid ourselves that the birth tourism thing is what conservatives care about... People doing that kind of thing are usually rich. The real target are poor illegal immigrants giving birth in the country.)