Without clarification of exactly what they said, it is possible they were referring to light braking to simply slow down without stopping, rather than hard stopping or panic braking. Personally, I don't use the front brake unless I intend to come to a full stop. To slow down, I reduce the throttle to let the engine slow the bike or apply some rear brake.
Yes, the front brake does 60 - 80% of the braking work. Considering that the weight transfers to the front when braking, using too much or only the rear brake can cause you to lose control as the weight is transferred off the rear wheel.
The rider needs to apply brakes progressively (some call it staged braking) to stop without locking either wheel. This takes practice and skill to be able to do it in a panic stop. These techniques are taught in rider safety courses. Some people practice in an empty parking lot to be proficient so react more naturally (automatic) in case of and emergency stop.
Sources of additional information:
Motorcycle Factors: Braking
Motorcycle Braking Tips