That's an interesting thought by Calvin, but doesn't answer the basic meaning of the term. Also, I think he's not assuming that this passage is discussing the Covenant of Works; he seems to be referring the impartial rendering of God to be to the believer; I don't think this passage can give this meaning, as the overall thought seems to be condemnation, and why the Jews are guilty under the Law's sentence, since they have not kept the law.
As to the understanding of the passage, it is undergirt by the truth that there is no respect of persons with God, verse 11; hence it cannot be something which is distinctive to the covenant of works, but must also apply to the covenant of grace.
But however the passage is understood, it is still an absurd inference to deduce merit from reward because it is a matter of fact that God rewards every man according to his works. Matt. 16:27, "For the Son of man shall come in the glory of his Father with his angels; and then he shall reward [render to] every man according to his works."
Verses could be multiplied.
They could, and they would all show that context determines whether the concept of merit is to be read into the term.