bunyip

Actually I’ve discovered that the answer is simply that the focus keyphrase isn’t even used for SEO.

You can see for yourself in the Yoast metadata on a web page if you right-click on the page and select ‘View page source’. The ‘Meta description’ is there, but no keyphrase/keywords. So there is nothing there for SEO audit services to pickup.

From what I’ve found online, the focus keyphrase is simply a way for the Yoast plugin to ‘rate’ your website – but it doesn’t help with SEO.