This same over simplistic view is used to explain why Russia expanded so aggressively to the west and east, and holds its client states so tightly.
There is a nice grassy plain from Poland all the way through to the Caucasus (with the Urals as a somewhat permeable barrier). Armies on the move love grassy plains.
If your objective is to protect Moscow, you need to control that grassy plain.
Edit: ref: Prisoners of Geography: Ten Maps That Tell You Everything You Need To Know About Global Politics by Tim Marshall.
"Geography is destiny" can be a useful lens through which to view global politics.
Pripyat Marshes are not a big stretch of the North European Plain. You can move massive armies through the northern route along the Baltic coast, or the southern route through Ukraine.
They're not really a big obstacle, given the two land invasions that came from the West in the 20th century.
Last I checked, the Germans crushed the Russians in WW1 while treating that front as a defensive action while the real war was fought on the Western front.
The Polish invasion in 1920 was pretty effective too until it wasn't, same for Napoleon, etc. Point being its a big corridor for large troop movements.
There is a nice grassy plain from Poland all the way through to the Caucasus (with the Urals as a somewhat permeable barrier). Armies on the move love grassy plains.
If your objective is to protect Moscow, you need to control that grassy plain.
Edit: ref: Prisoners of Geography: Ten Maps That Tell You Everything You Need To Know About Global Politics by Tim Marshall.
"Geography is destiny" can be a useful lens through which to view global politics.