This is 100% true. Some people have better traits for trading and it cannot be taught (Or it is hard for someone not good at it to be very good at it). I won't say its impossible but it takes practice and its hard to change, like for someone introvert to become extrovert or someone bad tempered to be calm.
I know Richard Dennis did a bet where he said he can get anyone to be a good trader. But in actuality he actually screened for people who innately have the ability to be good traders by asking questions and filtering them. Also, for those who made it and traded his system but deviated he dropped them off.
So by saying he can get anyone to be a good trader he was wrong. He actually screened thousands of people, picked the one with the best traits and then in the actual trading process kick out the ones who don't follow his rules. If he say picked 10 random people without screening and actually asked them to trade and cannot kick them out halfway, i'm sure the results will be different.