You are absolutely right. I admit with overwhelming pride that I have never taken any chess classes
In Chess, both players have perfect information over the game. In Civ (...) we do not
If you knew how to play chess you'd know it's actually WAY harder to be a good chess player than to be a good CIV player. There are very limited changes to what CIV can give you: who's your neighbor, which resources you get, which city states are close, etc. And it takes very little time to learn what to do on each case to adapt. It's definitely way easier than adapting to the extreme way in which a different opening changes a game of chess.
We are not meant to have perfect information over the game
Yes we do, the only difference is that it will take us a little time to get it. The part of the game in which we explore while being mostly "blind" to the map is very short. During most of the game we do have most of the available information about the map (especially the land around us, which is what we need). During the late game we know the whole map.
What I meant by your behaviour (save scumming)
Nope, I'm not talking about save-scumming. That would be re-doing decisions mid-game, or maybe replaying when RNG doesn't go your way (CIV 1-4), which might create the bad habit of not playing along when the game gets hard (which I certainly don't recommend). I'm talking about playing the whole map again, from turn 1, after you lost. Especially if you had the (frequently erroneous) idea that there was no way to play the map in a better way. Confirming that there was a way to win, and checking what it was you didn't figure out during your first attempt will definitely make you a better player, I have experienced this myself. I don't know any good CIV 4 deity player that hasn't used this method to improve. Now of course CIV 4 is harder than 6, so it takes more time to beat average maps on deity, but if someone is having trouble with CIV 6 I don't see how the same method won't work.
Your circumstance was you didn't know who was next to you
Wrong. If you read the whole thread (which you should do before replying to it, I may add) you'll clearly see that the save file I played is from turn 6, AFTER meeting Gilgamesh. This means the map can be won AFTER you know who's your closest neighbor (so it's not "unwinnable"). At this point you also know which units will take place in the fight, and before they get to your city you have already scouted all the tiles where the fight will take place. So if you know the map where the fight will take place, and you know the units... It's really very similar to chess. If you had never played the game you might argue you don't know how many units the AI will produce, but the truth is we know how many warriors they have when they start, we know they build at least 2 more, and we know there will be at least 2 archers (usually more) if they don't have a good UU (in which case you should expect about 2 of that UU and usually a smaller amount of archers). We also know that once we destroy the units forming the first invasion (usually no more than 7) everything will be easier and the danger of an early loss will be gone. If there are more enemies (as it happens here), we also know that the closer enemy is the biggest threat. There really aren't so many options.
the solution is how do you properly behave when you don't know who's next to you
If you have a close neighbor you pretty much always meet them before finishing your first slinger, they won't be hiding exploring only in the opposite direction to your land. So you'll always know they are there soon enough to start an appropriate strategy to defend from an early attack. The only decision we take completely blind is the first build, which we already know on Deity should be a slinger (or a scout on CIV 5, or a worker on CIV 4...).
The fact is that if you play this map over and over until you win the early wars, the next time you get a double or triple DOW against you before turn 20 you WILL know how to deal with it.