I think it is more that it does not thicken up when cold, like a straight 30 weight oil would.
I do know that at -30 you can not get a 30 weight oil out of the bottle. I have been running syth. 5-20 in these for years, never had a motor open since the 1960's I remember one time , the block heater breaker went out, and in the morning it was -45., truck started O.K., thou it was not happy. My little 50hp tractor would start when cold , if plugged in, but the hydraulics would not work below -10 unless run for a while.