It's simplistic and lacks nuance. Russia in the spring and early summer is very muddy, and would slow down an invasion more than the winter, and autumn turns to winter very quickly, so there's only really about a month and half of the year that you could actually "invade", but it is borderline impossible to actually organise and carry out an invasion of such a massive country in such a short amount of time.

It's spoken by a lot of these guys as a Truism rather than an actual examination of Russia's material conditions.