We certainly share the position that there is an acute problem of overpopulation, but this is not the source of the anti-natalist thought.
Anti-natalism is not a stricter version of claims about population explosion. Anti-natalism is a principled moral opposition to the creation of other people regardless of the number of people in the world, or the number of people that people plan to produce. The inherent problems with creating people are valid and exist in any reproduction, regardless of the amount of reproduction by other people or even the same people. Of course, the more children someone produces, the more moral transgressions they commit, but it does not mean that if someone produces far fewer children than the norm, it cleanses them morally. Every human creation is wrong. Those who produce a lot simply commit a lot of moral transgressions.
To make do with creating one person is making one moral offense, not refraining from committing one.
Arguing that one should settle for one is like saying hey I could have done a lot worse but I chose to do something bad only once. Of course, the possibility of doing many bad things is always open to people, but it cannot absolve the moral offence caused once, certainly not such a significant moral offense, and certainly not one that can be easily avoided.
And of course, the creation of one person does not amount to harming one creature. Each person harms many other creatures during a lifetime, and each person may produce more humans who will produce more humans and so on. And everyone will be harmed and will harm many others.