The hire man knew his stuff - he was offering sound advice to his market, probably in the hope of getting his car back in one piece.If you did your learning on an automatic, left foot braking will be natural. BUT if you are a long time manual driver, when you get into an automatic, tuck your left leg out of the way. The first time you set off, everything will work fine. A few times down the line, your body will forget and you will find that when you press the clutch, (which is really a great big brake pedal), you really kick it, and everybody on board is now sitting at the front.
Almost as bad is a habitual auto driver going into a manual. It happened to a work mate. He drove a Triumph 200 Mk2 Auto, needed to borrow my works van (HA Viva) for a trip up the road. All was OK until he stopped in the yard at the other end and wondered, briefly, why it was just bouncing up and down and spluttering. Then he remembered the stick thingy and that other pedal.