A full flight is not overloaded. All aircraft have very generous safety margins built-in.
Every seat occupied is no less safe than 1/2 full.
That must be why I was pulled off a regional flight. Turboprop in that case. I'd been scheduled for a later flight, but could make that one. They cleared me on, and then decided that they were *overweight*. That plane had several empty seats, and it only has about 30. Another time, this time on a regional jet, they asked for volunteers to take a later flight, because they thought they were *overweight*.
It all depends on how much they have in the baggage compartment, how much fuel, and what the temperature is. (DFW can get kind of warm in the summer, although not compared to Phoenix Skyharbor)
"Every seat occupied is no less safe than 1/2 full." That is such a ridiculous statement, I simply wouldn't know where to start correcting it. Just damn, dude, think it over a while before you make a statement like that.