I think he means RWRP, not out of phase.
I believe the middle was RWRP to avoid hum when it was combined with the neck or bridge.
Edit: from fender’s signature srv Strat spiel:
“The original Strat had three single-coil pickups, so it was able to produce sounds that no other guitar could deliver - particularly once players discovered they could lodge the 3-position blade switch in those in between positions to access neck, middle and bridge, and middle pickup combinations. These were often erroneously referred to as the out-of-phase settings. “