statement is only partially true on a number of levels. The idol is based on the Dumbarton Aztec idol of Tlazolteotl, the Aztec fertility goddess. Reynolds adapted the statuette to make it more picturesque for the film. For one, it is smaller so it would be easier for Indy to grab it. He also (obviously) enlarged the head so that it would look more distinctive. The statue was also gilded so that it would look more like "treasure" though, remember, all that glitters is not really gold.
However, if you dig a little deeper into the "original" Aztec (not Incan or even truly Chachapoyan) idol, you'll find that all that is old, well, isn't. According to curators and scholars, the statuette (which was acquired in the archaeologically fruitful and lucrative 19th century) does not seem completely genuine. Either it is completely unique Aztec art (possible) or it was an original statuette "modified" in Europe to make it more marketable or... (gasp) it is completely fake. When it was first written about in 1899 in France, it was lauded as one of the treasures of pre-Columbian Mexican art. It passed hands a few times before it was purchased by Robert Woods-Bliss who left it to Dumbarton Oaks, the research/estate he and his wife founded. A few decades ago, some scholars began to raise concerns, but obviously they were not loud enough to completely dissuade the filmmakers of Raiders to look elsewhere. Because, real or fake, it is extraordinary.
Which is basically the theme with all the Indy props and even movies. Who cares if the legends are a little messy and the objects are so improbable: those films are extraordinary entertainment. They are creative and daring and fun and smart and everything that movies should be. If I can be entertained, I don't care if what's entertaining me is real or fugazi. Or based on something that might be real but probably is fake. And I will claim that the little Aztec/Incan/Spielbergian/Reynoldian idol is far more worthy of pursuit (real or not) than the little golden man that puts the Hollywood tribes on their knees in homage. But... that's just me.
Sources (click for link)
"Intriguing Story...Stone Idol..." from The Washington Post by John Kelly
"Raiders of the Lost Ark Fertility Idols... Update 2013" from Original Prop Blog by Jason DeBord
"Raiders of the Lost Ark Fertility Idols in the Marketplace" from "Original Prop Blog" by Jason DeBord