Pope Shenouda III
If the commandment of baptism had existed in their days, they would have been baptized. But this commandment is laid down in Christianity. Why? Because baptism is dying with Christ and Christ had not died in the Old Testament.
The prophets of the Old Testament practiced what they could in their days: the symbols of baptism such as circumcision and crossing the Red Sea. They also celebrated the Passover lamb, which symbolized the Blood of Christ. We should not expect people to have obeyed a commandment, unknown to them, as it succeeded their time/s.