Chain wear due to standing while pedaling

Chain wear should be roughly proportional to the transmitted power. You can either achieve that power with high rpm and low torque or low rpm and high torque (e.g. when pedalling out of the saddle).

The amount of force required to plastically deform a steel chain is much more than a human can exert, even if you embedded the rear wheel in concrete so that all the force went into stretching the chain, rather than turning the wheel.

Chain "stretch" is 100% caused by wear, with grit in the chain grinding down the rivets and sideplate openings. Standing up to pedal has nothing to do with it. If the chain is dirty and its parts are moving relative to each other, the parts will wear down.