Of course it is relevant. If it is not capital, then it doesn't matter. It's like capturing cracow, when capital is warsaw. Or beijing when capital is nanjing. Capital is allways better defended than rest of the country. IE napoleon did not took petersburg.
Oh man I guess if USA got into a war and NYC got captured it wouldn't matter...Also he didn't take St Petersburg because Moscow was a more important target even though it wasn't the capital.
Also he took Moscow when Russia was much stronger. When Poland occupied Moscow it was basically on beginner's mode with cheats enabled so it doesn't really matter anyway.