Joseph:
The difference is that with Rashi’s daughters, the first mention of it is hundreds of years after it happened. R’ Rakeffet, on the other hand, knew R’ Aharon’s mother personally, as he mentions in the shiur (though I did not quote that part), and additionally he says that you can confirm the details with R’ Aharon. That would be very much going out on a limb for someone who is making up a story.