No, the Great Depression had a huge role to play in unwinding inequality. I'm pretty sure Piketty talks about this in Capital but it's been a while since I've read it so I'll give you the benefit of the doubt. In any case, it mechanically works the same way as war: a catastrophe incinerates capital, poor people suffer more in real terms but rich people suffer more in nominal terms and it's the nominal terms that drive the burden of furnishing runaway capital with runaway returns so the nominal wipeout has the very real effect of resetting the inequality spiral.

> every other major economy on earth was decimated

That would be a more convincing argument if the decimated economies didn't experience the upswing in middle class living standards as well. Gary Stevenson has been covering this from the UK angle and his story (and the UK's story) is the same: middle class flourishing in the wake of WWII and a steady erosion back to the mean. This suggests that the capital reset, increase in worker bargaining power, and the concessions won through it were causal while being on the fortunate side of the Atlantic was not.