In the general scheme of things you're right that many jobs are near linear, but I think for the intended audience of TFA (people likely to be scammed by the salary system and made to work longer than 40-hour weeks without overtime pay) it's a valid generalization.
The jobs that actually are linear actually do tend to get paid by the hour and often even have 1.5x pay for overtime, and the fact that salaried workers don't get that is precisely where the scam is.
> Nobody does.
You are making an overgeneralisation, or assuming a context. For lots of jobs productivity is fairly linear with time - some of those jobs work on piece rates or hourly wages.
Some people are productive for 9 hours in professional jobs, and they are billed accordingly, although maybe the marginal productivity per hour falls.
Not everyone is “wasting time, busy working, and showing off” to paraphrase you.