I dont really see what the big deal is. When I was learning to drive my dad taught me to use right foot for brake and throttle even in an auto, I asked him why and he just said it was a good habit and you're less likely to screw something up... so the next time we drove I tried left foot braking and it was no biggie, first time it wasn't very smooth, but by the 2nd and 3rd time it was fine. Though I went back to right foot braking straight after that because that's what my dad was teaching me

But I dont see what's so evil about it. Though one time when I was little I saw a car accident where the driver panicked hit the throttle instead of the brakes in an emergency and someone told me that's why it is safer to use one foot for throttle and brake, though that was a long time ago I dont really remember it, lol.
What was more dangerous was the first time I drove a modern small car instead of the 25-30yo medium to large cars I'd driven up until then, the brakes were so different I was jerking the car around until I got used to them. Or driving in socks, that was far more dangerous than left foot braking