Not having actually related test data to back it up, I can't say for certain. Some studies have been performed on irrigation water saving, especially in Australia. This cost-based study with similarly operating chemical films cites Witting (1998) as stating that evaporation losses (mass) could be reduced by up to 50% in small storages. This report would indicate that the disadvantage of chemical based methods in large water storage is their inability to reduce convective losses (unlike a pool cover). However the study of heat transfer methods would indicate that some reduction in heat losses is possible. Check the wikipedia page for the basic elements of heat transfer here
Evaporation is fundamentally a cooling process like sweating. Heated molecules get enough energy to move into the air, reducing the total energy in the pool. If you can find a chemical that can stop this (most oils would probably work) and that would be practical to put in your pool without killing you or ruining any equipment (and won't break down with your chlorine), then logically that chemical would stop the heat that would go with it, (with the caveat that the consequential temperature increase could allow other methods of heat transfer to compensate partially.)
From the "Heat Transfer" wikipedia page you can see that conduction and convection are proportional to the temperature difference while radiation is proportional to the difference of the fourth powers of temperature (so it becomes more significant at higher temperature difference. Evaporative cooling is slightly more complex however its potential is proportional to the difference between dry bulb and wet bulb temperatures (see here).
The important question is how effective would it be and that depends on various factors that will influence the temperature:
- the depth of your pool (how hot it gets sitting in the sun),
- the surface area,
- how hot you like it etc.
For instance if you like your pool to be 25C and you live in a tropical (high humidity) environment then it won't take much to stop evaporation. If you live in Alaska and you like your pool at 30C then evaporation might be a big part of the losses of heat, but so might convective losses too (from air) as well as radiative losses.
To answer your question, and interpreting the above references, I imagine that something like this could be effective in a special case of moderate, dry climates for people who like their pool 'moderately' warm. This may be where they get their "50%" energy saving (although I'd like to see it!). If you like your pool really warm then the other components of heat transfer would become bigger players in the total losses of heat and evaporation would be less significant. I imagine that convective heat transfer from the air plays a pretty big part in cool and more humid climates which means your product would be less effective. Evaporation would play a bigger part in any dry climate, which your product may reduce.
Lastly, if you estimate that evaporative losses are significant to you (in absolute non-relative terms) it is probably because your pool is in a either a low humidity climate, or you like it warm, or it is exposed to high winds. Most low humidity climates are cold and those that aren't cold are often so hot that you wouldn't care about heating your pool. So in just about all the cases mentioned a 20% to 50% reduction in evaporation losses would only represent a small portion of your total heat losses. The old proven technology of the physical cover, according to references cited, appears to be the most effective at total loss reduction.