His ethical guideline has nothing to do with how close humanity is to extinction.

Except I already wrote:

You can easily augment the underlying harm avoidance principle with a condition that it should not result in the extinction of intelligent life (assuming that intelligent life doesn't cause even more harm in the long run).

You don't even have to apply the principle of charity, you could just look at what I had literally written.

However, if practiced diligently, it can bring humanity to extinction in a few generations from any population size.

Nonsense. Most humans don't live in a warzone at any time now. And followed in extreme poverty, this principle would reduce local malthusian traps and probably reduce poverty; at least the suffering of children from poverty.