You need to reread that. It's world economic growth that makes gold impractical.
An inability for most people to assay gold purity "in the field" also would cause problems.
It is far easier to vet the validity of paper currency on the spot (I can use a little marker to eliminate 99% of fakes) than it is to validate the purity of a high value metal like gold, particularly when the gold the isn't available in common, government minted forms.
This is to say nothing about the historical problem (the solidus and denarius for example) of government abasement of metallic currency, which disproportionately affects high density gold coins, in which a tiny variation in actual gold content with a constant weight can drastically affect the value in "real" terms. Silver currency, while more commonly abased, was at least less of a loss per coin.
