This seems to say that forests were gone, so all tree dwelling birds died out and thus all modern birds are descended from ground dwelling birds. Looking at the paper [1] it seems to only say 'reduced flight capacity' which makes more sense, considering we only know of 4 times flight has only evolved so this would be a big deal if birds had evolved it twice.
1. https://www.cell.com/current-biology/fulltext/S0960-9822(18)...