This method of doing transparency in mirrors is objectively inferior to the mod's. Many shader creators have spent a lot of time and work on traying to create very good and workable transparent mirrors. There are a number of methods, but most of them require a bunch of extra complicated work with the shader to make them viable for faded alpha to work - hence the common technique of using black pixel cutout.
The huge problem with this mirror is that EVERYTHING that is rendering in transparent renderqueue cuts out of any meshes that are behind them, and in some cases completely invisible. Blushes on avatars, and transparent hair bangs will always cut out an avatar's face and this is very jarring. This effect is NOT desired, and a great amount of effort was put into avoiding it.
Certain HDR transparent materials go negative in the mirror as well.