> in proportions of less than 1 part per million.

How much less? I believe most gold produced in the US is from ore with under a half ppm gold (E.g open pit mines in Nevada).

Maybe the point there is that we already have practically endless supplies of quarter ppm ore ready for the taking on the surface of the earth. Gold is rare only in so far that the current price reflects the breakeven point of these most abundant sources. Adding more supply with similar or worst production costs wouldn't change anything.

All the other precious metals are less than 1 ppm compared to iron, but platinum is more abundant, and by weight it is about 6 ppm in iron.

The advantage of an asteroid is that its entire metal core has 6 ppm of platinum and a fraction of a ppm of gold, while on Earth the quantities of ore containing such amounts of precious metals like a half ppm or a quarter ppm of gold are much smaller.

There certainly exists no "endless supply" of gold ore with a quarter ppm gold, because the average concentration of gold in the crust of the Earth is a few parts per billion, so the few places where the concentration is as high as a fraction of a ppm are compensated by vast areas where the gold concentration is much less than one part per billion.

While an asteroid may have a lot of iron containing 6 ppm of platinum and a little less than 1 ppm of gold, that is not comparable at all with a terrestrial ore with 1 ppm or a few ppm of precious metals.

The precious metals are the easiest to separate from rocks, which is why one can exploit on Earth ores with a so low content of metal. On the other hand, precious metals are very hard to separate from iron, which is the very reason why in any planet or asteroid these metals end up being dissolved in the iron core.

So the extraction of platinum or gold in so small quantities from iron would be extremely expensive on Earth and much more so on an asteroid, where it is impossible to produce most of the chemicals used on Earth, like acids or cyanides.