Is opening an unplayed save and performing any active actions within it considered maphacking? Yes, it is.
As for whether maphacking is a cheat or not—everyone can decide that for themselves. In this case, we are talking about creating an advantage (even if a negligible one), since Alex has more time to make a decision. And if the Dragon Utopia had been a minimum-level area and could be broken by a quick battle—then Alex would have seen what it gives (since that is exactly what he intended to do—check the contents). Alex did not cheat in his own understanding. Perhaps, from Medium's point of view, this is also not cheating.
Essentially, I would like to hear the community' same opinion on whether it is permissible for one of the opponents to open an unplayed save at all. And another thing—what exactly is cheating?
Otherwise, with similar logic, one could conclude that if you drank a bottle of vodka but didn't want to drink it (and no one forced you)—then you remained sober...

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Therefore, I decided not to continue this game.
I have no intention of refusing new games with Alex, provided that he agrees with my position regarding the rules.

Then—what prevents continuing this match? After all, Alex "did not gain an advantage"... You would just keep playing...