Paradise means a walled garden, not necessarily heaven. Christ did not go to heaven on good friday, so paradise must refer to being in christ's presence and the good thief descended with christ into hell, which christ turned into a paradise by his presence. It stands to reason that the good thief would also be ressurrected and baptised.
Additionally, Hell was divided between the Limbo of the Fathers and Gehenna. Christ only went to the Limbo of the Fathers; He did not free the iniquitous dead. The Limbo of Fathers, being a place for the virtuous, could be called Paradise.
In any case, whatever Our Lord meant, we know He could
not have meant Heaven, as it's dogma that He descended into Hell.