I don't think that really answers the question. That women's and men's brains show, on average, some differences isn't really in dispute. That those differences are sufficient to explain why women less frequently end up in STEM subjects is very much in dispute. Because, you know, in between birth and career choice there's a lifetime of societal factors that will also play a part.
Incidentally, that's not to say that the idea of "disparities in recruitment doesn't imply the company is sexist" is bogus -- but the employee seems to have missed everything in between.
However, the opinion itself is neither here nor there. He has a right to express it. Google has a right to say that such opinions aren't welcome in their workforce, since they would, I am sure, far rather not have to deal with the negative publicity that effectively sanctioning the idea that "women just aren't cut out to be engineers" would bring.