First, The New Covenant did not apply to the thief on the cross. He lived under the Old Covenant. Second, Jesus had the power to forgive sins while on the Earth. Try again.
My sense has always been that baptism is a sign of taking on the new covenant with God.
It does not bode well for a supposed "believer" whose first act as a Christian is to disobey one of the Lord's instructions and to reject the covenant sign.
I would call into question the legitimacy of their faith....in the absence of extenuating circumstances that made the baptism impossible or too hard to do at that time.
The thief was not under the old covenant. The old covenant ended with John the Baptist.
Does He not still have that power? Or is that a temporal function reserved for priests or something like that?