I don't think this would make a wet mess, except on roof mounted AC if the water output is greater than what would be evaporated on the condenser.
This unit appears to be designed for home units, which have much deeper condenser coils and therefore greater contact, read 'evaporative surface', than the small units on our trailers.
If you made your own, you would want to use distilled water to avoid the scale problem that they claim their filtration system would avoid. You would also need to control the flow so that the water sprayed on the coils would evaporate completely, preventing excess water which would drip from the unit, avoiding the 'wet mess'.
Not a bad idea really. Big commercial AC units use chillers which serve the same purpose. However, the principle is much the same as the old evaporative cooling units people used to put in their car windows
. They work, but the efficiency drops off as relative humidity rises.
With all the rain in Houston lately, I suspect the reviewer quoted on the site is not having as good results this past couple of weeks.