Too bad some pertinent pages are not included free, but I see from what you found that those folks possessed a gold-copper alloy that is said to be harder than regular bronze. Ordinary bronze when forged correctly is harder than plain wrought iron, and not too far behind forged iron, so although I haven't found good comparative charts yet it seems likely that the superior alloys in question would have been at least as good as, and possibly better than, those used by other ancient cultures whose bronze-age stone cutting technique is rather uncontroversially attributed by most to well trained human craftsmen. Of course we can't know for sure what bronze they used, but it is at least certain that they knew hot to make it.
Here's a link on
Inca Metallurgy, which cites successful modern experiments in the use of bronze and stone tools, along with abrasives, in the kind of work we're talking about: