Study of other civilizations shows that the transition from keeping records and cultural artifacts (religious doctrine and practice, epic tales, etc.)orally to writing everything important down occurs over a significant period of time IF the writing system must be invented internally.
This doesn’t mean that the civilization lacks development or detailed concepts. In fact, these civilizations are often amazingly advanced when they first begin writing down their activities. This is the case for the Egyptians, the Sumerians, the Harappan civilization of the Indus Valley, the Chinese, and for the earliest civilizations of Central America.
The earliest written records we have from these civilizations show exactly the governance-related activities that these two scholars are reporting; census, taxes, land records, etc. Once a system of writing is established, there are also signs of broader diffusion when non-government entities, primarily merchants, begin to utilize writing in very focused ways to support their activities. By focused, I mean it is not uncommon to read of merchants who can read and write just sufficiently to run their businesses but are otherwise functionally illiterate. Later, as the writing system becomes more developed and its use more widespread, scholars emerge and strive to capture cultural knowledge out of concern for its loss as the “oral tradition” fades and the population of “professional rememberers” begins to pass away.
The young scholar may be in for a frustrating search. The apparent lack of extensive narrative records in the Inca system may simply mean that the internal demand for the Inca writing system to capture more complex information had not yet sufficiently developed. The Spanish conquest, by supplying a fully developed language system that the colonial administration insisted on being used for record keeping and other writing purposes, probably brought a dramatic end to its further development. The Incas, at least those with the ambition and intellectual capacity to adapt, did what colonized peoples often do: learn the language, customs, and culture of the colonizer and use it, when and where they could, to win back their lost freedom over time.
The Inca/indigenous accounts that the scholars seek probably do exist, written down quietly and circumspectly in Spanish by Spanish-speaking Inca administrators and intellectuals. They are going to show some bias but this is to be expected. The writers had to live in their own time and operate within what was possible given the rules of their reality. They also had to accommodate themselves as best they could to the synthesis of two distinctly different worldviews, Inca and Spanish, that was occurring.
There is little evidence of any long, gradual development of writing systems. The earliest surviving examples of a script are generally readable, and the full-blown system has picked up sophistication and elegance in a few generations. The Incas were only around for about 140 years.