I disagree that it is meaningless. It is always good to understand your operating envelopes e limits. The calculation is obviously the upper limit of ms hours that are available to the economy. You will never get there because, as you point out, people can be students, between jobs, opting to stay home and raise kids, etc. but it would be an interesting stat to track for trends.
I don't see what this adds to the conversation that you can't get from the labor force participation rate that we're discussing. Obviously Stockman is going for shock value but the number doesn't tell us anything useful for making policy.
If he had a serious economic point he would state what he considers "full employment" under his scenario. It's obviously well over 20% unemployment just to account for students and stay-at-home spouses who aren't looking for outside work.
I said it's meaningless because he doesn't tell us whether his 43% number is good or bad, let alone how close to full employment it is.