I think what he is referring to is that the left turn signal often means it's clear to pass a vehicle.
We live on a fairly busy hilly road with lots of beach traffic. We have to make a left turn into our driveway/garage. We have learned to, yes, use the left blinker, but also to put our left hand out and give the "slow down" signal. Also to really really watch the cars behind us!
Generally, driving in Mexico is easy. Different, but easy. Little things like the left blinker, but as safety goes, if there is a hazard ahead of you, on go the flashers immediately. If you are on a two lane road, oncoming traffic will flash their lights and wave their arms if there is something ahead, be it a cow, a slow down, whatever. Never see those things done in the US.