1. The Taney Court was just reading aloud from Article IV, Section 2:
[No Person held to Service or Labour in one State, under the Laws thereof, escaping into another, shall, in Consequence of any Law or Regulation therein, be discharged from such Service or Labour, but shall be delivered up on the Claim of the Party to whom such Service or Labour may be due.]
2. That said, the Taney Court was inexplicit, the dictum that black people held as slaves were not people but property was gratuitous as well as doubtful legally and brutish morally, and it would have been better if the Justices, rather than wandering around all over the lot, could have rendered a simple opinion that was at the same time concise, harmonious, and less offensive to people's sensibilities.
3. That said, Dred Scott illustrates the folly of people's having acquiesced in John Marshall's power grab, which as someone's helpful quotations from John Taylor in extenso, supra (q.v., loc. cit., and all that), show clearly was just that, an unconstitutional grab that established a point that had been carefully considered and rejected twice by the Constitutional Convention in 1787. The absence of judicial review from Article III is thus explained as deliberate, and Marshall's assumption of it exposed as unconstitutional, and treasonous.
4. All of which goes back to the continuing argument, whether we are better off with the Supreme Court as the "final" arbiter (not as long as the People sign their checks, say I: the People are Sovereign) of what is and isn't constitutional throughout the Union, or whether we would be better off with the States reserving final judgment to themselves, or agreeing to an Amendment specifying how constitutional review should be accomplished.