That is of course true. The key word is winter equipped, troops used to fight in harsh climates (like the Siberians were). To them, the severity of the winter around Moscow was probably something they were fairly used to, and although the troops themselves might have been more or less on par with the rest of the Red Army, being rested, well fed, well equipped, and acclimatised to the weather conditions would put them leagues ahead of the opposition (in this case being the luckless Germans of army group center).