Who is "email"?
You are right. I do not know whether he was baptized previously or not. He could very well have been immersed previously according to John's baptism, later apostatized, and finally repented on the cross.
Personally, I have found it difficult to make that point effectively and thereby persuade others. In my opinion, I have chosen to grant the assumption that he was never immersed. Admittedly, this may be a more generous approach, but I do not want someone to feel like I have not squarely addressed their question.
This has merely been my personal approach. The approach you mentioned would be equally valid and significant in dealing with any attempt to establish the thief's conversion as a pattern for modern conversion.
Thanks! May God help us to have a sincere love of the truth,