How do you respond when people bring up Mao Zedong killing sparrows and causing a famine that killed "millions"?

My reply is that yes, it was a top-down mistake, government policy was a major factor compounding the famine (the CPC even admits it) and nothing like that has happened in the many decades since, demonstrating the ability of the government and the broader movement to learn from mistakes and avoid repeating them. As some other replies pointed out, their history of regular famines has stopped entirely since the Three Year Great Famine, so they have clearly learned, improved and overcome a major hunger issue in a country with huge food demands.