Mothers with son's DNA in their Brains?

In this NYT article, the phenomena of chimeric and mosaic genomes are discussed. Mention was made of autopsy results on the brains of about 60 women where they found that more than half had the Y chromosome in their neurons. The article goes on to say that "They most likely came from the sons they all gave birth to."

I don't see how this is likely b/c immune tolerance during pregnancy is a local temporary phenomenon. In addition, you would have to ask yourself how did a male cell find it's way into its mother's brain and start functioning as a neuron. This is not inconceivable but goes against Occam's Razor more than a chimera created during gestation.

I believe it's more likely that the mother obtained the Y chromosome while she herself was in HER mother's womb, and the source of the DNA was a male fraternal twin that only lived long enough to populate his sibling's embryo/fetus. The foreign DNA would have been tolerated because immunity only kicks in after all differentiated cells have come into being.
I wonder how hard it would have been for researchers to determine double-X in male brains? Did they do it? I can't afford the darn journals to find out.

Also, I would need to ask if there was a quantitative estimate of the Y chromosome content. This is important because more than one or two cells would indicate an origin during development whereas a few cells would support DNA coming from a gestation where a male was being carried.