I don't think the argument is that you need to "have proof". Its that the work you do during the weekday isn't amassing you long term value. If you lose your job/want to do something else, you have minimal "credit" beyond the ability to hold down a job to apply elsewhere, and any provable skills you've gained in the process. Writing code outside work isn't the only way to gain "credit", just one way of several that he mentions in the piece.