He's used rebar which is made of low carbon steel (Between a 0.2 and 0.3% of carbon while high quality knive's steels have more than 0.6%). Low carbon steels have a very low response to thermal treatments, so they need a very fast quench to have a noticeably increasment of hardness. Normally is better to use oil to avoid obtaining an extremely hard and brittle knife, but with low carbon steels quenching in oil wouldn't have any effect. Although water allows a faster quenching, the hardness achievable is low. Low carbon steels are good for practising but they're not suitable for making useful knifes
