GW190521's Luminosity Distance is greater than the age of the universe - how?
Luminosity distance is simply the distance that the object would have to be at in a static special-relativistic (or Newtonian) universe to have that luminosity. It's not the distance in the expanding general-relativistic universe that we actually inhabit.
Out to distances of roughly 1 billion light years, spacetime is roughly flat, the Hubble speed is not too large, and the luminosity distance is not too inaccurate. At substantially larger distances, it's way off.