Which ethnicity's population are we going to reduce? How will the social mechanisms work to reduce population? Who will hold that authority to dictate things and how will it be enforced?

Interestingly it doesn't seem like those questions need to be answered, since the birthrate is dropping all by itself for some reason.

Which ethnicity’s population are we going to reduce?

I honestly believe that "we" aren't going to do jack s**t. It's a process which is nearly unsteerable. People are going to live longer and longer, and use resources that would otherwise be used by children they might have had. Society is going to be burdened by caring for the old, and this is going to reduce chances of caring for the young.

In nearly every developed country, population growth is slowing or population has already started decreasing. Only in the least developed regions (some areas of Africa) does the opposite still apply, but UN predictions (made by competent people) suggest the process just reaches there later.

So, every ethnicity's population is going to be reduced. Every ethnicity can also consider if their numbers are adequate, too high or too low. If a nation feels threatened by disappearing from the maps, they can try to reorganize their society. Random ideas: a few laws that give parents various health and social security guarantees regardless of their employment status, especially in case they're single parents, then maybe create a few dating sites that actually try to help their users find people they like, etc...

You are arguing a strawman. Both the article and me are talking about people voluntarily chosing to not have children. I don't see anything wrong with that, and neither with promoting the idea that this is totally ok.

And no, it isn't just the top few thousand. Even if those were gone tomorrow we would still have very similar issues realistically speaking. But sure, limiting the excesses of the top few thousands would also help and is a politically reachable goal. Solving the over-consumption of the top 2-3 billion needs an strong change in mindset, and politics alone will not be able to do that. But at least many of these 2-3 billion are already getting few children voluntarily.

It was meant to be humorous framing, given the impossibility of making humans from magma or the iron core. :)