I was expecting to see major points in Russian 20th history

Why were you expecting that? What led you to think that?

The film is pretty much about Russia's relation with Europe, from the founding of St. Petersburg (the "western-facing" city) to the drawing back from Europe and eventual isolation that followed the end of the monarchy. The ball segment at the end is even explicitly referred to as "Leaving Europe". Although that's by no means the only possible organizing theme, it does seem to me to be internally consistent, coherent and interesting.

And the film is most definitely about The Hermitage (including the Winter Palace part). We see historical events that in some way intersected with the art and architecture being displayed. For example the ceremony of the Iranian apology actually happened in the very room where it was recreated. (There are scenes that refer to later significant events at the Hermitage too: WWII [making one's own coffin during the seige] and the Stalinist period [insufficient funding worrying the director].) In a sense the last events in these buildings that were significant to all of Russia were the ball (no more balls after the Revolution) and the arrest of the Provisional Government (in the room where we see Tsar Nicholas' familly having dinner). After that the seat of government moved to Moscow.