It's not "building on" in the "estimate of all land built on" sense. It's putting windfarms in existing agricultural land, where the farmland remains but has windmills in it too.

[1] gives a range of around 5MW/km2 to 20MM/km2, which assuming 10MW turbines is roughly 1 turbine per square km.

[1] https://web.stanford.edu/group/efmh/jacobson/Articles/I/Wind...