Food security. Not agricultural autarky.
But like you said, we can all believe what we want to believe. Though it is humorous how similar these arguments are to Western think tanks about China’s food imports being a “
”.
One doesn’t have to dig deep to see the root causes of most famines or food shortages. It’s almost never because there were “too many mouths to feed”.
But even adopting a policy of maximalist births would not necessarily create a larger population. No developed country really succeeded in having high fertility without mass immigration. Whether China should have mass immigration or not, that's another issue and I think some degree of demographic level immigration from friendly nations is desirable.
Family planning policies suggested most families, especially the urbanites to only have 1 child but this did in reality not have a great effect on fertility. When the commission dropped its policies, it had next to no effect on Chinese birth rate for almost 15 years.
What it did and was meant to achieve was the popularisation of contraceptives in a society that earlier did not have a modern concept of their use, and this is obviously a very important thing for developing the modernity of the country. A sharp drop in birth rate only happened a generation later, precisely due to the spread of contraceptives and improved sexual health.
In early transitional economies which China was in the 1970s (as well as in developing countries), people generally do not value their children as much as they should. This type of thinking is very damaging to the future of the nation, because quality workers, not quantity is what makes a country great. Africa and India have quantity, yet you'd be hard pressed to even find a single screw let alone a high tech device made there. That's because manufacturing is largely a winner takes all with the most efficient and educated workforce taking over almost everything.
China has also far outpaced the projected growth during the Deng era. IIRC Deng expected China to become the largest economy in the 2050s, in reality it happened in the 2015s. So the projected population carrying capacity back then would not be reflective of today's carrying capacity. Basically most Deng era projections are invalid.