This excerpt might be a little misleading. 70°F is not a magical number by any stretch. Given time, a person without exposure protection will get chilled if the water is lower than body temperature. Being comfortable temperature-wise in the water entails wearing the appropriate exposure protection for the amount of time one intends to spend in water at a certain temperature.
Being chilled, or feeling subjectively cold, implies that physiological blood shifts are occurring in the body -- blood shifts that others have already explained. On a theoretical level, it is believed that blood circulation is involved in the inert gas on-gassing and off-gassing processes which occur during a dive. If this is the case, it's not a huge leap to infer that blood shifts in the wrong direction during a particular phase of on-/off-gassing may increase DCS risk. There may also be certain molecular "middle men" involved. One hypothesis is that cold stress can increase concentrations of stress hormones in the bloodstream, which may heighten DCS risk. Another hypothesis is that
cold stress can increase platelet activation giving rise to a prethrombotic state leading to DCS.
That being said, the U.S. Navy has conducted extensive trials to show that, given the conditions they describe in a population of fit, young males, a "chilled" diver has an increased risk of DCS. Many of these studies on cold stress in divers have been published. Feel free to do a few searches on the
Rubicon Research Repository.
