It is really whisky.
Normal whisky is made by brewing beer, then distilling it to remove water to get it to about 90% abv, then diluting it down to 40%.
Freezing beer has the same effect, the water forms ice and can be filtered off from the liquid, which becomes super concentrated beer.
I do wonder if its unavailability is due to the interest of HMRC. When beer is made duty is paid on specific gravity of the mash, which is a predictor of the final alcohol content. When whisky is made duty is paid on the % ethanol in the distillate, so a much higher rate.
If the makers were paying beer duty and selling what is in effect a distilled spirit then HMRC would have been "unhappy".
You can try this process at home. As students we used to try it. Pour your choice beer into a plastic jug, freeze it and remove the ice crystals. Drink what is left.
It is usually horrible.
