They are referring to their world’s equivalent of the famine of 1932-1933. Read more about it on Wikipedia and at UCLA.

Aeron left home when he was about 14-15 years old, around 1929-1930, and if Eva was born in March 1918, she ran away from home at the age of 14 in the spring of 1932. Since this is not a historically accurate story at all, and this is not the real Soviet Union of our world, let’s say that the famine happened two or three years later. (Or, we could say that the revolution and civil war happened a few years earlier, so that Eva would be born in March 1914 (year of the tiger) …)

Of course, Oleg died long before the famine, so he can’t know any more about the reasons for it than Natasha and her mother. But since he was a convinced Communist (he was among the about 20% of all cossacks who served on the red side in the civil war) and also a bit macho, he has difficulties accepting how it could happen …