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.