DNA gathering legal once arrested

That’s why it’s not a substantive rebuttal.

Well, the assertion a reliance on fingerprinting is a problematic analogy because the Court has never directly decided the issue of whether fingerprinting to solve a crime is permissible is a substantive rebuttal. I am not sure how exactly or why you think otherwise.

However, J. Scalia made several points in his dissent and to deride his dissent as “not a substantive rebuttal” on the basis of, ostensibly, a single point you disagree with does not make sense.