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.

Fascist Authoritarianism is a form of government and communism is an economic system, communism has been used as a carrot dangle by corrupt fascist authoritarian governments many times but has nothing to do with communism itself, and even so fascist capitalism has caused way more far reaching harm than fascist communism has

Yeah, but it was the last famine china has had. With over 1800 recorded.

Despite the damage and death caused to humanity and the sad extinction of multiple bird species, Mao didn't come up with the sparrow campaign out of nowhere. Scientists genuinely believed they were a disease/parasite vector. Its no different from how countries today cull populations of pest species, inoculate animals that spread disease, and control animal populations during disease and hunting seasons.

I always say that for those in a position of power, its better to do something than to do nothing. Mao and the scientific community didn't have the full picture and couldn't see the extent of all the potential consequences, the goals were good, and the communist party and scientific community took steps to mitigate and learn from mistakes.

Neoliberals Getting mad at Mao for allegedly killing millions (which Monthly Review expresses serious doubt over) and blaming communism and mao is like getting mad at early humans burning themselves to death when they first tried to use fire to cook food or keep themselves warm.

If the Chinese government should be fully responsible for starving millions, then it would be no less logical to give it credit for saving millions, too:

During the Difficult Three Year Period the state also expended large amounts of money and materials carrying out famine relief in the heavily stricken areas. Spending on relief funds was increased, the basic food supply for disaster victims was guaranteed, large teams of medical care workers were sent to provide medical aid in the disaster-stricken areas, and so forth. With mobilization and direction from the CPC and the state, the entire nation fought together against droughts, floods, locust plagues and other natural disasters, using a variety of production and self-rescue activities.

(Source.)

Mention the wording of "affecting millions" does not equal "killing millions" as a starting point

All you should need to say is there were famines every couple of years before the communists came to power. After they came to power there was a single famine. There were no more famines after that. If communists really wanted to cause famines wouldn’t they have allowed/forced even more afterwards? They didn’t, and the lack of famines also showed that socialism eliminated famines. And now they’ve eliminated poverty striking a strong blow to general food insecurity too.