Earlier quoted context omitted.
I think this comment is based on some confusion about how languages spread. Languages spread along with people, but while a local language may be replaced, the people are not generally replaced with the language. There may have been some genetic mixture, there may have been a time where they were conquered by them for a time, but there's no sense in which the people who wrote those works _were_ Yamnayan, any more tha…
All the recent palaeo-DNA data suggest a horribly massive process of genetic replacement of the local population by the new arrivals. This process is of course very uneven -- e.g., the population of Ireland seems to have mostly shifted to a new IE language -- but in some cases the change was drastic. Moreover, in some parts of Europe this seems to have happened several times, with first agriculturalists replacing loc…
How much do English-speakers today know about the events in early 10th century France that eventually led to English becoming a sort of pidgin French, full of words like "eventually" and "sort" that didn't exist in Beowulf? How much effort do they typically devote to passing on traditions about Æthelwold's challenge to Edward the Elder in Wessex?
And that's after 1100 years of a literate, mostly settled culture with libraries that contain physical books from that time, in a culture that values that kind of factual knowledge of history, rather than more practical sorts of knowledge such as how to properly worship Agni to gain his favor and which plants to poison your arrows with.
Oral tradition can preserve knowledge to an astounding degree. There are songlines, as I understand it, that record the geography of landforms that have been undersea since the Ice Age (https://www.scientificamerican.com/article/ancient-indigenou... roughly the same time as the Proto-Indo-European culture). But it is hardly surprising when it is silent on a topic we wish we knew more about.