Not per the Roman Catholic definition of penance.
The thief on the cross was being executed for a crime he committed against the Roman Empire.
His execution had nothing to do with his confession...other than he realized the hopelessness of situation and appealed to Jesus for forgiveness.
To what "Roman Catholic definition of penance" do you refer?
Etymologically, the word "penance" comes from a Latin root meaning "punish" or "punishment". Pretty sure crucifixion would qualify.