That makes no sense at all. If he died under the law then he would go to hell not be with Jesus in paradise - he admitted he was guilty of the crime for which he was being executed and deserved to die. His repentence and acknowledgement of Jesus for who he was is what obviously got him into paradise. He was saved under the New Covenant which Jesus had instituted the night before.
Abraham, Moses and the Prophets also died under the law. But they are in Heaven - why? Because of the grace that was made available to all due to Christ’s death on the cross.
And the thief died and was saved BEFORE the fulfillment of the Law. Just like Abraham, Moses and the Prophets.
He was NOT saved under the New Covenant which Jesus had instituted the night before. Read Acts 1 and 2. Only AFTER the death, burial and resurrection (Pentecost) did Salvation come to all who believed that Jesus rose from the dead and then repented and were baptized (Acts 2:38).