I looked around just a bit and am, admittedly, puzzled! Could find no actual tech data, patents, etc. Making the ads come across as cryptic, even to the point of seeming evasive or simply ill-informed. If I were still at work, with access to spectrophotometers, etc... would certainly run traces!
With that background... I am just guessing that it comprises traditional angle-adjustable crossed linear polarizers with integral dye filtration. It sure seems ingenious, if it really does (efficiently) what they seem to be claiming... especially at the dirt-cheap price!
NOTE: The extinction of traditional crossed linears is undoubtedly wavelength-dependent. It might be that is really all they are doing, just putting a wavelength scale on it, rather than the more-traditional ND scale... The more I ponder this... the more convinced I am. That idea would actually be patentable as "novel use of an existing device" --- quite possible patented fifty or more years ago?! Tom