AI cheating on single-player maps.

This is more of a no than a yes. Because you can't overcome water reefs by walking.

Exactly: it's flying! And the fact that the AI even spends mana on it ("like an honest player") is surprising! I didn't know that (though Loins had so much mana anyway that it wasn't a problem).

Could you upload the map for review?

Yes, I can. The map is interesting, I repeat. Link:
http://heroesportal.net/maps.php?search=longest+journey

Added after 11 hours 44 minutes
Addition. I continue the "Longest Journey" walkthrough. I defeated Solmyr (who also flew more than once) - among his artifacts, there are neither Angel Wings, nor a Wizard's Hat, nor a Tome of Air Magic, nor a Scroll of Flight. And the following week, he appeared in the tavern. I hire him: and he doesn't have Flight in his spellbook! This is proof #2.