what does “ending world hunger” mean?

Distributing agricultural surplus at market rate relative to population demand rather than market demand.

Would not food security lead to higher birth rates

Firstly, no.

The higher the degree of education and GDP per capita of a human population, subpopulation or social stratum, the fewer children are born in any developed country.[

Pulling people out of starvation tends to reduce family sizes, as people don’t plan their families with the expectation of high levels of child mortality.

Secondly, “you need to starve to death because we’re afraid you might live long enough to have kids” is a fucked public policy on the scale of Israeli genocide in Gaza.

Finally,

lead to higher food requirements, when sometimes it already feels somewhat unsustainable?

Sustainability is a consequence of land use policy, not population rate. India and China are the classic case studies of this in practice. But you can see the pattern repeated across the planet.

Vegetarian agriculture is significantly less taxing on the ecology than animal agriculture. When you compare arable land requirements per Ethiopia, Bangledish, or Thailand residents to the dietary demands of Americans, Israelis, or Argentinians, what you discover is the enormous toll animal farming takes.

The unsustainable clear cutting of jungle and near-malicious misuse of limited irrigation drives up costs and cripples availability in even the wealthiest (and most thinly populated) nations on Earth.

Meanwhile, significantly more populace regions can thrive on a primarily vegetarian diet.