there is somthing called coo you know , yeah he will outspeed them but not able to toch him is an ExaggerationIf two people with a strength of 8 and 7 fight a person with a strength of 10. It is not necessary that they would win even though they're combined strength is higher.
Luffy's speed in G2 is so vastly superior that both Zoro and Sanji couldn't touch him.