I think you misunderstood his comment, though admittedly the word "expected" was probably unclear. He said "might". In other words, if the market price is $38 and you place a limit order at $40, you may get $38, you may get $39, you may get $40.
So if the price is moving (upward) quickly, you will get a price in the range you implicitly specified, but you may not get the price you had hoped for.
I dont think that long (buy) limit orders are ever meant to be used that way, you would place a long limit order below the market price and it usually ends up being executed at that price even if the stock nose dives a lot more
So if the price is moving (upward) quickly, you will get a price in the range you implicitly specified, but you may not get the price you had hoped for.