I haven't read McEwan's book, but it seems like he may have been a little hyperbolic in order to set up some reveal about all the other incredible things the Inka (or the Andes) had that allowed them to establish such a large territorial empire. The phrase "supposed handicaps" reads that way to me.

Though the population of the Andes didn't follow the format that our modern perspectives normally identify as "writing", they did develop ways of recording, transmitting, and reading information. Information was encoded on knotted strings known as khipu/ quipu. Using various types of thread, typically made from cotton (in more coastal areas) or camelid hair (in highland areas), information could be encoded through the use of different knots, colors, patterns and materials. By hanging additional cords or threads off a "primary cord", the khipu was able to organize information hierarchically. Obviously, this is great for keeping accounts of tribute payments, troop numbers, etc.

While early scholars thought that these collections of knotted string were more like an abacus than a book, it's clear that khipu contained more than just numerical information. Some people have argued that khipu were essentially mnemonic devices, designed to aid the memory when interpreted by a specialist would fill in the gaps. These specialists, known as "khipukamayuq" or khipu keepers, were often called upon to read khipu in court disputes, which we have documentary evidence of from the Spanish colonial period.

Contrary to the mnemonic interpretation, it seems that there are certain consistencies in documented khipukamayuq readings that would indicate that khipu contain at least some degree of structured narrative information. Obviously, interpreting khipu through court documents recorded by Spaniards comes with many challenges. Not the least of which is the multiple steps the information passes through from the khipukamayuq to the Spanish recorder. Anyone who's ever played a game of telephone will know that the more steps any information passes through, the more likely it is to contain errors or inaccuracies.

One promising area of research at the moment is working specifically on collections where multiple khipu can be matched to a colonial document to look for links in the information encoded in knots and in text.

Medrano, Manuel, and Gary Urton. “Toward the Decipherment of a Set of Mid-Colonial Khipus from the Santa Valley, Coastal Peru.” Ethnohistory 65, no. 1 (January 1, 2018): 1–23.

Alternatively, the emerging possibilities of big data and computer databases may allow us to find patterns in large collections of khipu, sperated from their context, that scholars have so far had difficulty engaging with productively. https://khipukamayuq.fas.harvard.edu/ This is a promising area of research, and it requires the involvement of mathematicians and computer scientists alongside historians and archaeologists.

Other Sources:

D’Altroy, Terence N. The Incas Second edition. Chichester, West Sussex, UK ;: Wiley Blackwell, 2015.

Urton, Gary. "From Knots to Narratives: Reconstructing the Art of Historical Record Keeping in the Andes from Spanish Transcriptions of Inka Khipus." Ethnohistory 45, no. 3 (1998): 409-38. Accessed April 21, 2020. doi:10.2307/483319.

Edit: capitalization