Not all of the human thought process goes on inside the head. An engineer with a computer is far more productive in terms of designs generated than one with a pad of paper (and in turn more productive than one without any tools whatsoever).

You are right, and it's interesting to consider this quote from the article in that light:

At some point our AGI will be just as smart as the world's AI researchers, but we can hardly expect to start seeing super-fast AI progress at that point, because the world's AI researchers haven't produced super-fast AI progress.

What would a group of human AI researchers capable of completely reimplementing a copy of themselves be able to do? I'm assuming for example's sake that if an AGI could do it, so could the human researchers it is on par with. That's actually a tremendous amount of power for either an AGI or a group of humans. As it is today we've been lucky to discover modern medicine and farming techniques and find fossil fuels just to boost the total population and scavenge the tiny percentage of scientists and engineers off of it. We won't be able to double the number of high-quality AI researchers every 50 years for long on this rock without an actual improvement in the rate of growth of AI research. The point where any system acquires the ability to be self-sustaining seems like it would have to be an inflection point of greatly increased growth.