ILS26L,
In a nutshell, what you were told was correct. The hotter it is, and the higher the elevation of the airfield, the more degraded the take-off performance is for any aircraft. This means that to achieve satisfactory take-off performance, a compromise must be met where either fuel or payload is left behind. Since there will be a minumum fuel requirement on the Flightplan that must be carried, that only leaves the payload, which on this occasion was unfortunately YOU! This is why generally you will find that "hot and high" airfields have long runways, in the case of Mexico City 12795ft or 12966ft depending on the runway in use.
PHX