The way TLJ is written, the whole “your parents were nobody” was all Kylo trying to turn her. So it being the unvarnished truth doesn’t make a lot of sense.
Rey’s the one who says it though, not Kylo.
But Kylo gets her to say it. He is goading her. He may have seen more in her memories, but in good dark side fashion, he uses only what will work for him.