Completely disagree with the statement.
when you trade in demo, if you use a indicator or EA, same signal or trade does not execute in live trading. because demo is a demo, but live account involved realtime execution, chnages made by brokers to the prices, slippage, execution delay, liquidity issues, etc etc.
Therefore, demo is good only to get hands on trading.
but NEVER trade in a demo and apply the same in live and 99% of the time you will loose.
If you do not believe me, open a demo and a real account and apply the same strategy in 2 accounts simultaneously and you can understand what i am saying here.
ALL THE BEST for trading.