Is fasting on Taanis Esther considered as if one drank Ad Dlo Yada on both Purim and Taanis Esther? Just like we find eating on Erev Yom Kippur is considered as if one fasted on both that day and Yom Kippur.
Yes, it is considered to be the same thing.
Just as you can experience cognitive handicaps when drunk (so much so as to confuse Arur Haman and Baruch Mordechai!), you can also do the same thing under dehydration.
Beware that according to this line of reasoning, the minimum and maximum shiur for fasting on Taanis Esther are probably the same, if not reversed in their order. But hey, we tried :)