I agree that the Old Covenant had the gospel preached to Israel in many shadows, types, and promises, but the Old Covenant itself did not administer those benefits of Christ to them. That was not the nature of that covenant, it had a completely carnal existence. It could only preach about another covenant for true righteousness, true forgiveness, and true reconciliation with God. But the covenant itself reaffirmed the Covenant of Works and conveyed only temporal promises to Israel.
"Since humanity brought itself under the curse of the law by its fall, it pleased the Lord to make a covenant of grace. In this covenant he freely offers to sinners life and salvation through Jesus Christ. On their part he requires faith in him, that they may be saved, and promises to give his Holy Spirit to all who are ordained to eternal life, and to make them willing and able to believe.
"This covenant is revealed in the gospel. It was revealed first to Adam in the promise of salvation through the seed of the woman. After that, it was revealed step by step until the full revelation of it was completed in the New Testament. This covenant is based on the eternal covenant transaction between the Father and the Son concerning the redemption of the elect."
- LBCF 7:2, 3
Before the New Covenant arrived, the Covenant of Grace was not formally given, only announced and promised (revealed). The Covenant of Grace is one covenant revealed progressively and concluded formally under the New Covenant.