Hi Evil Nemesis
Interesting technique though touchy.
The issue is the technique. The tutorial says " This technique works best on scenic photos, such as of trees, leaves or gardens, but feel free to try it on any images you want." And that is true. If you look at the image in the tutorial it too changes the highlight areas. The areas you might want to frost are not always the highlight areas.
Indeed the upper layer is monochrome and then the lighten blend indicates that take the highest value for each channel separately and use the larger value.
To help zoom in to what you want, leave the Layer Blend to normal and play with R,G,B and constant until the areas you want "frosted" are brighter than the areas you don't want frosted (thats what I did) and then turn on the Lighten blend and fine tune from there.
I found a combination of sliders that might be closer to what you wanted yet I had to play around with it a lot to get that result.
Hope this helps yet I am not so sure if it will.
John Wheeler