What do you think it could be?

I haven't looked at the save file; I'm not interested. I've seen controversial save files, and after the second or third one, they all became boring. It's impossible to prove anything with 100% certainty. There's always room for chance. That's the first thing. Secondly, a player might be playing completely recklessly, and where you see malicious intent, cunning plans, save file analysis, map analysis, etc., etc.

You really should have looked at the save file (in the first post) or at least read the posts (to see what others think).
The thing is, in these two duels with him, there are many suspicious moments that fall under the categories of "gut feeling," "intuition," or "luck." But this phrase made me laugh:

it could just be a simple oversight; he went there because he clicked there with the mouse.

This isn't the level of play or the type of player who would just "click randomly with the mouse."
When playing offline for a result, can you play perfectly for a couple of days and then suddenly "click" randomly?
It's just hard to explain when you haven't seen it and don't understand the essence of the issue... there's no random guessing of the direction of movement; instead, there's an obvious and well-chosen plan to take away the main hero's army, "whittling him down" before the supposed (but not obvious) battle, buffing and speeding up the unit that will have a favorable terrain, and creating a questionable (in terms of cost-effectiveness) chain of castles to deliver a small but, as it turns out, sufficient number of troops, all directed towards an absolutely unattractive and unpromising direction, the end of the map with no visible prospect in case of an unsuccessful choice of direction... and there, it turned out to be a necessary defensive structure and a portal to the "treasury."