There's a fundamental difference between arguing that human population is not a factor at all (which it is, as has never been in dispute), and between arguing that it is not a *major* factor. That seems to be a subtlety that is either being missed or deliberately ignored.
The point is that if, say, we collectively switched our electricity supplies to cleaner sources, improved farming methods, had more energy-efficient houses, cleaner fuel supplies for transport, changed diets towards eating foods that are less of a contributory factor, and so on -- all of these are choices that can be made and improve the situation without needing to tackle the population issue. All of them can be assessed to be more of a factor.
That is the point. There is no dispute that alongside this, tackling overpopulation is worthwhile for its own sake, partly because it would also go hand-in-hand with a general improvement in healthcare, women's health, and other numerous benefits. But to argue that this is the only, or the major, problem to focus on in order to tackle Climate Change is a position unsupported by any evidence I'm aware of. It's oversimplistic too, and perhaps it;s also an attempt to absolve Western nations, and their citizens, of their own responsibility for the present epoch of Climate Change.