In some of the other charts over the years similar to the one you’ve posted, I’ve read that the 300lbs passenger weight is included in the payload. Some don’t specify. Always wondered if that’s true or not.
The 300 pounds would be for the j2807 towing (GCWR) only.
Payload is always GVWR - base weight. Base weight does not include driver, passengers, or cargo, but does include all the options shipped from the factory (including stuff like running boards and tonneau cover if your truck was shipped from the factory like that). It also includes full weight of fluids/fuel.
So take your truck from the factory, fill it full of gas. That's your base/curb weight. Now take your GVWR and subtract base weight, that's your available payload. Every pound you put in, including driver/passengers, comes off the available payload. GVWR doesn't have to do anything with towing, though it might, it could just be you putting stuff in your bed. Its a separate standard from towing. But if you do tow then you have to include the tongue weight from the trailer too.
When it comes time to tow a trailer we also need to stay under GCWR; GCWR = gross combined weight rating, that is the weight of your truck + trailer added together. The J2807 standard allows 300 pounds for your driver/passenger.
It does get a little confusing. 300 pounds has to get deducted from your payload (GVWR) if that's what you and your wife weigh together, but your GCWR allows 300 pounds so that first 300 pounds doesn't get deducted for GCWR.