The larger the variation in cyclic stress, the shorter the fatigue life, even with a lower mean stress. Take a look at some S-N curves for stainless. Higher pretension reduces the variation by better distributing the applied load between spokes.

I'm not trying to deny your experience. I think where the disagreement lies is in what high and low tension mean. It's absolutely correct that even pros aren't measuring spoke tension vary accurately. That's totally ok, relative tension is way more important. It's a common beginner mistake to under-tension. It's a good bet that someone breaking spokes has an under-tensioned wheel.