Extending the hitch and extending the trailer tongue both have the same effect on the truck and increase the leverage the trailer weight places on the truck. It’s like using a long breaker bar, you aren’t pushing any harder but the force exerted on the bolt is much higher.
Extending the receiver, yes, you're keeping the pin weight constant, but extending it further back, thus creating greater bending moment (about the rear axle as your pivot/fulcrum).
Extending the tongue does the OPPOSITE. Your hitch ball is at the exact same location as before - about 8" past the receiver. However, a longer tongue arm DECREASES tongue weight. Think about it, your trailer's CG is constant, your trailer axle(s) is your pivot/fulcrum. But now you've extended your moment arm (distance from tongue to trailer axle).
Therefore by extending the tongue and not the receiver, you're putting less load on the truck frame.