To quote Leithart in full:
A strict distinction between law and gospel is offered as a prophylactic against works-righteousness. If it is admitted that law is gospel in any sense, all is lost.
But this view assumes the very same view of law that it contests. A proponent of works righteousness sees the law as demands that need to be kept, and the end of the law is simply its keeping. The proponent of law/gospel segregation has the same view of law.
Both detach law from its purpose, which, as Knight argues, is eschatological. Law exists to correct, instruct, lead to maturity. And to that extent, law is always intended as, and in practice often is, good news.
However is this to misunderstand what is properly meant by Law/Gospel. Charles Arand and Robert Kolb in their conversations on the Book of Concord note that even "Christ died for your sins" can be "Law" rather than Gospel if you hear it as condemning you for the magnitude of your sins rather than lifting you from despair (they said it in a really menacing and accusative way to emphasise the point).
Kolb and Arand don't make the connection but I wonder if the same cannot be true for commandments. Commandments can be good news if they bring life rather than condemnation. I think of the beatitudes which Matthew characterises as Gospel (Matt 4:23) although most people teach them as commandments - do this and you will receive this (be merciful and you will receive mercy etc). However, if read in context they seem to be saying: “rejoice because you who are X are going to be saved”. It does not say be X, but assumes that they already are. Equally though it could be preached as an impossible standard we cannot reach in seeking justification by our works.
So the same words can have the different meanings for different people. God knew this so the Law of Moses was intended to be both life and death to people, just as the Gospel is (2 Corinthians 2:16). This may make interpretation of the different statements in the bible a whole lot easier (although it may be cheating).