If you open the plugin file in a text editor, you can see all the settings associated with the preset. In the case of this one, I can find the following:
VignetteAmount = -100,
VignetteMidpoint = 0,
Which suggests that it's not using 'post-crop vignette' but the lens correction vignette instead. You could verify this by applying the preset, and checking the value under 'Lens correction > Manual > Lens Vignetting'.
In my opinion this is a poor choice, as applying the effect to a cropped image would only apply vignetting to the uncropped version, so e.g. if you cropped the bottom right off the image, the top left would be darker than the other corners, making it look odd.