Genetically, modern humans are almost identical to our ancient counterparts; it’s not a difference in evolution that has allowed us to grasp continuously more complex concepts, it’s the accumulated knowledge that we have gathered and passed on. Without being saturated with that information from a young age, a person would only be as intelligent and knowledgeable as humans were well over 100,000 years ago.
Sure, yinglets may not have the same capacity for intellectual development as humans (as far as anyone knows), but the bigger issue is that their society has only recently moved out of the stage of basic survival, so they’ve hardly had the resources and manpower (yingletpower) to assign to things like studying and archiving the results.
Also, the problem that the elders are worried about is a common one, which has occurred on Earth every time that a more developed culture has met a less developed one. “Borrowing” things from the more advanced culture would allow the less developed one to develop quicker, but almost always results in their society becoming more like the one that developed the information in question.