This is a false statement. IB paper trading environment always place limit orders at the end of the queue, not at the front. So it simulates the worst condition.But one needs to understand the mechanics of the fills with the different systems. IB Paper is "top of book" fill on the add liquidity side. Which means you jump to the front of the order book que. In real markets there are people ahead of you, MKT orders, and then the other people with LMT orders that submitted before you.
.
Ex.:
bid - ask
0.50 - 0.75
If you place a limit order to sell@0.75, the order will get filled only when the market will move to:
bid - ask
0.75 - 1.00
meaning when no other orders will remain to sell@0.75.
IMO, demo account is synonym of paper account.