That is a weird expression to be making in an advertisement, which usually wants to emphasize conventional attractiveness. A team of people decided that this is the shot they wanted to go with, so there has to be a reason.

Also, my earliest point of contact with pornography was the late 70's, when me, my mom, and my sister would be outside a store visiting the city waiting for my dad to come out with something in a brown paper bag... so I don't think it's inconceivable that the art team would have had some exposure given the time frame and the fact that who knows where the DeBeers advertising team was even located.