They took the slavery / serf distinction seriously in the Slavic world. While the distinctions might have mattered in the 1800s, by any modern-slavery standard the serfs were slaves, as you rightly say.

Whatever we might now think with hindsight of the communist revolution in Russia, if ever there was a country primed for the peasants to rise up against the ruling class, it was Russia.

Russia might have been a land of art, music, dance and literature, but you were pretty fucked if you weren't in the 20%.