Freeze distillation is cheaper and more controllable for small quantities of stuff, IIRC. Also, of course, you'll get different mixtures of volatiles (and thus different flavours) with different types of distillation; they may have tried several and decided this was nicest. But I suspect it was easier and cheaper :)

You could fortify it with spirits, but in order to get from even a very strong (say 10%abv) beer to this strength, you'd wind up with something that contained more spirit than original beer. So it might not taste quite like beer any more, since it'd really be cheap vodka diluted with a bit of beer :)