I'm perfectly happy in my job, does that make Birkman Express irrelevant to me?

Not really. "Helping people who aren't in quite the right job identify why that is" is definitely one of Birkman's strengths, but that doesn't exclude people who are already in the right job. Far from it: it is better to invest in improving what you can already do well than to focus on what you can't do. By the same token, the person who is already in the right job can benefit a great deal from understanding why that is such a good fit. Understanding that means you can focus your personal development on those areas of existing strength, and aim to do more of what you were born to do and be.