I read the study. The "problem" with it is that it only examines biodiversity before and after abandonment, but without the abandoned areas being rewilded in any way.

Some areas are so destroyed by human activities that they need some help in rewilding, which is definitely possible. Of course, this study does point out that some management of the land that resembles previous human activities could help biodiversity. And of course, as the study mentions, they did not include "Mosses, lichens, mammals and invertebrates—including soil organisms such as nematodes and molluscs".

Keep in mind also that biodiversity is difficult to measure can be counterintuitive. Farmland and managed land creates edge habitats, or a confluence of habitats, where measured biodiversity is higher. Of course, when that edge habitat disappears the measurements can go down. But that does not imply that merging a huge number of farmland back ot a more contiguous wild are won't support biodiversity better in the long run.

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