Actually the basis for concluding that the thief was not baptized is very strong. John's baptism took place about 3 1/2 years before the crucifixion. All of Jerusalem and Judea went to hear John, and many repented and were baptized.
Since those two criminals continued with their crimes during that time (and were eventually apprehended and crucified) it is evidence that neither of them repented and was baptized by John. So the repentant thief on the cross was saved PURELY by grace through faith. Which totally refutes your false doctrine.
THere is every possibility that the thief had been baptized by John, was a disciple but later fell away into a life of crime until meeting up with Christ on His cross.
From the discourse that took place between Christ and the thief in Luke 23:40-43:
--the thief believed in God, knew God existed and God was to be feared
--admitted his guilt being repentant
--knew Christ was an innocent man
--knew that Jesus death on the cross would not be the end of Christ
--knew Christ had a kingdom and saw the need to be in that kingdom
The thief had a better knowledge of Christ than some of Christ's own disciples. Maybe he had this knowledge because he was once a disciple himself.
--There remains no definitive proof the thief had or had not been baptized.
--He is not an example of NT gospel salvation
The thief argument continues to fail on many points.