Is Birkman a skills inventory?

No - the online questionnaire can't tell whether you have acquired certain skills or not. Instead it looks at how you are wired with regard to what motivates you, what you think is important in behaviour and communication, what is likely to stress you and so on. Based on a huge reference sample, it then compares your profile to those of people known to be successful in certain fields. Birkman is never saying "you have the skills to be a top lawyer" - but it could be saying "if you wanted to apply yourself to acquire the skills, you certainly look like you have all other characteristics of a successful lawyer" - or whatever it is. And when you think about it, acquiring a skill is usually the easy part of the equation - skills you can train in.