This site says that " (exchanges do not attach bitcoin miner fees)...Bitcoin exchanges often try to send their transactions as efficiently as possible. This can mean that the fee is not large enough or doesn’t arrive quickly enough for a miner to include your transaction in the next block."

What does it mean by "send their transactions as efficiently"? Do they just send transactions without attaching fees? Somehow the latter statements, which imply that exchanges do attach bitcoin miner fee, contradict with the content in the earlier bracket.