Magic or Education?

It has an indirect effect. Since the chance to block a spell depends on the level of the enemy hero, it would be very useful if Zoltan's level is higher. Thus, these 15% of experience can play a significant role in the final stage.

You are very stubborn, there are no words! Let's add "Logistics" here as well - it also has an indirect influence on the growth of the hero's levels, and, following your logic, on Zoltan's specialization. Don't forget about Summoning magic either - it helps to defeat very strong neutral creatures without losses and also has an indirect influence on the growth of the hero's levels... We are waiting for new discoveries...