I define it practically: any mental state in which I'm able to score higher than usual playing Dual N-Back. Any more complex definition falling back on 'being able to suppress unwanted stimulus' or 'remembering in short-term or working memory that which is needed', seems too vague and begging the question for my taste.

I agree. And it's too bad - there's a lot psychology has to say about mental focus and attention, about how high working memory leads to greater focus, etc.

It looks to me like garden-variety armchair speculation and argument from personal anecdote.