Salt and CYA both mostly stay in the pool water although I suppose some of the salt is consumed. They are both mostly lost to splashout and backwash.
So... if the ratio of CYA to salt is 1.5/40 and your target CYA is 80ppm I'd surmise that the target salt is 2133ppm. That doesn't sound right. Maybe they're factoring the consumption of salt in this rule of thumb.
I'd still stick with dosing CYA based on test results and not blindly follow the rule of thumb. However, adding salt is probably a good reminder that you should check the CYA level.