That's not part of your claim. Let's stick to your claim that they knew Jesus was going to die for our sins.
That does not say anything about those in the OT knowing anything about Jesus dying for our sins.
Can you please provide evidence that everyone in the OT or the Gospels knew that Jesus was going to die for their sins.
Abraham understood God would provide a sacrificial offering, when God substituted a ram, for his offering of his son Isaac. Genesis 22:11-19
He did not know the details of crucifixion on a cross, as God's means, but he certainly understood that sin is only remitted by the shedding of blood, and all covenant promises are ratified by the death of an innocent.
As did Abel, who offered blood sacrifices to the Lord, with which the Lord was pleased. Genesis 4:1-4
Noah understood, and collected pairs of "clean" animals into the ark, so that blood sacrifices would continue, even through the judgment of the flood.
The entire tabernacle and temple ordinances, point through the animal sacrifices to the eventual "Seed" that God would provide, as the substitute in death, for the covenant children of God.
Jesus says all of the O.T. teachings, were concerning HIM. Luke 24:44-48
The only souls who did not comprehend these promises (gospel) of salvation, based upon the death of the Testator (God Himself) and the offering of innocent blood, were and are reprobates left to perish in their sins, according to the good and holy Justice of God.