He answers a different question:

So. I ask: how many people does it take, as a minimum, to maintain our current level of technological civilization?

In the collapse-and-rebuild scenario you don't need to "maintain the current level" right away. For example, you don't need to be able to immediately build contemporary computer-controlled cars. The fully-mechanical cars of the XX century would do fine, for a while. All you need to do is have enough technology to not get stuck in a local minimum and get the positive feedback loop going. That's a much easier task.

Of course by the time you're done with the rebuild, your 10m people will multiply :-)