I think the loss of traction they are referring to would be the skidding you would experience if you don't rev match that shift. And 6 - 3 would take a significant rev match. Agreed, 6 - 4 is a better choice IMO.
This is what I was thinking, made me facepalm if that was the case.
-2012 Grabber Blue GT 6M Premium w/ Brembos, Glass Roof, HID security pkg, Rear Camera, 401a, FRPP GT500 Axle-backs, Front Pioneer TS-D6802R speakers, JHR bracket, Bondra clutch extension.
-1986 Mazda RX-7 GX - Fast Mirage 16" Wheels, Sumo Firenza 205/60R Tires, slotted brakes + performance pads. Otherwise factory stock.
-2008 Mazda Mazda3 GT Sport - Stock, 5MT.
-2004 Volvo XC90 T5 - Comfort pkg, safety pkg.
-1995 Infiniti G20 - Sold, Pioneer head unit, lighting mods, miss it dearly.