While that's true it has very little to do with Mexico City's specific issues. It's at 7300 feet elevation surrounded by a ring of mountains that keep out a lot of the rain and moisture, and there's already not that much agriculture going on given the limited space.
I don't know anything about water allocation and provisioning in Mexico City, but the article itself doesn't limit itself to Mexico city either:
"It exemplifies a future that cities worldwide could face if global warming and overpopulation continue."
The city where I live is, I'm sure, very different from Mexico City, but we have reduced our water usage while the population has grown, thanks to things like xeriscaping. I imagine things are more difficult in Mexico City because there is less money, and orders of magnitude more people.
> we have reduced our water usage while the population has grown
I don't know where you live, but if it is a city like most others, you've likely reduced your per-capita water use, but not total water use. This is certainly true of most cities in the American west. They have made impressive (above 30%) reductions in per-capita water use, but they have (in many cases) grown by more than that.
