I don't think you can count "non-NATO" when NATO did not exist yet.
The point is that Finland matters a lot to Russia. The border with Finland is huge, geographically hard to defend, and right next to Russia's capitol. Finland's maritime zones join with existing NATO states to block Russia from operating in much of the Baltic (a process that would be completed when Sweden joins). Finland joining NATO is more than a shrug for Russia.
Moscow is nowhere close to Finland. St Petersburg is. Most of Russian border area near Finland is just forest.
Oops, fair enough—I had just been thinking of older history and forgot that the Bolsheviks moved the capitol. St. Petersburg is still the second-largest city and a major strategic location, but you're right that it's not the capitol.
Finland was also part of Russia until 1917.
Yeah, and the Finns hated it. My ancestors immigrated to the US to avoid being drafted from Finland into the imperial Russian army.