Do you mind linking to an article or video of her making that claim? There are ways to interpret that which are reasonable and ways that are not reasonable.

Broadly speaking there is no real way to perfectly explain what the monuments meant to the people who erected them and why they did so, because the cultures in question did not write their reasons down (although we do have somewhat temporally close oral evidence for Rapa Nui). It is also a general truism about monuments that they contain multiple meanings and those meanings can shift when contexts change. Consider, for example, the two monuments known as "Cleopatra's Needle": two obelisks erected by Thutmose III, which then had secondary inscriptions added by Ramses II (a point here is that Thutmose was about as distant from Ramses as Abraham Lincoln was from us--here we already have a shift of context from a testament to a mighty living king to a piece of venerated history), over a thousand years later they were moved by the Roman Emperor Augustus to adorn a temple to the imperial cult (the temple itself originally constructed for Mark Antony by Cleopatra, another shift in context) until the late nineteenth century, when they were purchased by private individuals and one was shipped to New York and one to London, where they stand today. What do these monuments "mean" then? I suppose some sense of a statement of power is retained throughout, but the power represented in a New Kingdom pharaoh and that of a Victorian parks commissioner are quite different.

Of course this sort of musing isn't particularly helpful to your question, which is about the people who first erected the monuments. Explanations for Stonehenge and the Moai do often revolve around commemorations of ancestral power (purposely not using the term "ancestor worship") and may have had some sort of memorial function. Probably the most cited explanation for Stonehenge, for example, is probably that proposed by the archaeologists Ramilisonina and Mike Parker Pearson that they were somehow a "place of the dead" based on similar monuments known to the former from his home of Madagascar. If this is correct (and it may well be) then it does have some sort of mnemonic function in that it reminds the living of those who came before--but I would caution against thinking of this as a practical memory aid. Whenever somebody proposes some sort of practical function be it calendar or memory aid or landscape marker it is worth considering the sheer scale of these monuments. There are many, many much easier ways to do something practical than lugging rocks over from Wales.