That is surprising. They have an inferior efficiency already due to the Betz limit (it is proportional to surface area facing the wind) and will always be about 60% max for a HAWT and worse for VAWT. However, the dynamics of turbulance have been one of the hardest things to model in large wind farms. The article is a bit light on details, but the last time I went to the Sandia Labs wind turbine conference in Albuquerque, VAWTS were nowhere to be seen (2015) and the biggest modeling challenge was large farm interferrence. Very interesting.

Betz Limit: https://en.wikipedia.org/wiki/Betz%27s_law