50/50 is not arbitrary. It's the average gender ratio.

"The problem we have however at the moment it that the current culture is trying to use Highly generalized bins to decide who should be doing what." I completely agree. In fact, Mr. Damore and Ms. Soh are using a highly generalized definition of what engineering should and shouldn't be and what characteristics engineers should and shouldn't have.

Then they argue since women (on average) are less likely to have those characteristics (because of biology) it's ok if they are underrepresented in engineering (and other STEM) fields.

So the problem with the whole "women don't like STEM because of biology" argument starts with how we define what engineers should and shouldn't be. We are defining an entire career and profession into a bin based on our biases of what engineers should look like and then saying if you don't fit in this box then don't bother.

That's a huge problem and addressing that is at the heart of diversity campaigns.