Absolutely true. I fly fish in MT in the winter. The first time I went it was zero degrees F and got down to -7. I wore too thick wool socks and my feet were about as cold as they've ever been. Now, I make sure there is plenty of room in my wader boots. Still, if my feet sweat at all, they get cold but not frozen no matter what combination of socks I wear.The trick to warm feet is opposite of what most people think. LESS IS MORE in 90% of cases. People regularly over insulate their feet . That is a guarantee of cold feet. The trick is to not let your feet sweat , period. Socks are much more important than boots. You need a good wicking sock against your feet and a good wool sock over those. Its also critical to have breathing room in your boot WITH BOTH PAIRS of socks. Beyond that, get the minimum recommended insulation for the Temps you will encounter.
This advise was pounded into my head by reps in the boot business from danner, rocky, muck, Irish setter, etc etc when I was in the sporting goods business. It works !