The text for this achievement reads: Successfully defend Cuzco without constructing any new walls or gates in the second Pachacuti mission “The Field of Blood”.
At the very start of the game, I deleted all my units. (I thought deleting all my units would complete the first objective of this map.) After deleting all my units, I lost the game and I still got the achievement.
I had a similar experience.
I deleted my starting units but didn’t loose. Instead it let me keep playing and I played along and realized I had messed up so I quit using the in-game menu. On the post game result screen I got the achievement notification.
I suspect the game checks if Cuzco was defeated and since the game ends with them (or the wonder) still standing you “won”.