Because the Bible says ... “... that if you confess with your mouth the Lord Jesus and believe in your heart that God has raised Him from the dead, you will be saved. For with the heart one believes unto righteousness, and with the mouth confession is made unto salvation.”
Romans 10:9-10
And since he said he believed in his heart that God raised Jesus Christ from the dead and confessed with his mouth - both of which I heard him testify to ... then he is saved, according to the Scriptures.
OSAS is a false doctrine. It doesn’t take into account the gift of Free Will that God gives to all mankind.