There’s no hook in WordPress that allows you to add a third option there. So you need to add a new option at the bottom as described in that post you linked to. Unless you add something using JavaScript, which I don’t recommend.
I think that makes more sense anyway. In the latest version of WordPress, the reading setting says “Your homepage displays …”. This is about selecting what to display on the front page, nothing more. Your homepage doesn’t display a portfolio page. It display either a static page, or the latest posts. Choosing a portfolio page has nothing to do with that.
That’s why I really recommend you to add a separate option at the bottom of the page.