You're probably right.

But just for fun, what if their actuaries found:

1. Very few men are willing to select non-binary even if it means a discount.
2. The women who would select non-binary are willing to pay a premium to be "affirmed."

Then they wouldn't pass up the free money by putting a third classification in between male and female. It's exact premium determined by the balance between #1 and #2.

Edit: I found a reference to OP's image. https://ghostarchive.org/archive/svTif

To my surprise, they found "non-binary" to be cheaper than female.

Daily Mail has confirmed the trend - finding the costs of a comprehensive policy for a 2016 Hyundai i30 was about $730 cheaper for a 25-year-old non-binary customer than a male - bringing the cost from $2,300 per year to $1570. The same policy for a female customer was about $1,917.