Flight to them is not equivalent to the Albatross or Condor.
A very similar 33 lb. Andean Condor's wing-span of 10 feet by 1.5 feet supports the same pounds/square foot in flight as a 158 lb. ancient Argentavis' wing-span of 25 feet by 3 feet.
So any suggestions that those old birds couldn't fly are pure sophistry.