Am I misunderstanding, or are you claiming that motivation is purely an inherited trait?

Misunderstanding. Expanding the context slightly:

I agree. I think it is quite obvious that ability is always somewhat heritable (otherwise we could raise our pets as humans), but this effect is usually minimal enough to not be evident behind the screen of either random or environmental differences. I think this applies to motivation as well!

It doesn't. (Unfortunately.)

When it comes to motivation the differences between people are not trivial. When it comes the particular instance of difference between the sexes there are powerful differences in motivating influences. Most human motives are related to sexual signalling and gaining social status. The optimal actions to achieve these goals is significantly different for males and females, which is reflected in which things are the most motivating. It most definitely should not be assumed that motivational differences are purely cultural - and it would be astonishing if they were.