the authors ... don't actually say what method was employed to remove the outliers.

You didn't cite the authors' paper :-(

A significant number of readings produce zero power. This likely corresponds to scheduled or unscheduled downtime where the turbine is disconnected from the grid or otherwise not generating power on a windy day.

Most of the other filtered readings correspond to low-efficiency operation. This could be caused by wind direction being off-axis from the turbine, deliberate use of a mechanical brake, poorly maintained lubrication, and similar mechanical issues.

Given that additionally a small number of outliers are discarded from the "unusually high efficiency" side, there's a decent chance the authors computed standard deviation and retained just those points that are within about 2 sigma of a fitted curve. This would explain filtering of the "zero power" outliers, as well.