You can't cheat in a single player game. Old World, an excellent historical 4x game, has an undo. It can be turned off in settings.
Of course you can cheat in a single player game, the difference between cheating in a single or a multiplayer game is that you are only hurting yourself and not any other people. And depending on personal preferences and philosophy you might not even be hurting yourself by cheating.
But the concept definitely exists.
Reloading a save game when you don’t like an outcome in a game that has been designed with this feature, is technically not cheating, but it is a quite similar concept to cheating nonetheless, since you are bypassing some or all game mechanics at your own leisure.
Computer games have a long tradition of giving players the option to choose to reload negative outcomes or not, and I think this has been a good solution since it both allows the people that want to use the save feature this way to do it, but also makes it clear for the players that they are bypassing all the intended game mechanics, so they may not necessarily be playing the game the way it was intended by the designers this way.
I strongly dislike the idea of an undo option to a strategy game. Of course it is their game and their design and not mine, but I hope that this kind of “muddying the waters” between reloading and playing the game normally, doesn’t become prolific in the genre. I think that ultimately would be bad for peoples enjoyment of these games.
On a similar tangent I dislike the addition of an “undo battle” option in the HOTA mod for Heroes of Might and Magic 3 even more, because that one will confuse new players of the game into thinking that the great designers of the original game intended that people do that, which was not the case at all. In fact I would think that a “perfectionist“ approach to getting battles exactly right would be very poisonous to the enjoyment of Heroes of Might and Magic.
A very different development, which I’m not opposed to at all, is what I’ve heard about the tactical battles in Age of Wonders 4, where you are supposed to be able to let the AI play trivial battles for you, but if you don’t like the outcome, then get one option to do it yourself. This doesn’t sound like something you could exploit much, just a great time saver in a game like that, so I’m all for that.