A couple of series inductors of appropriate size that will saturate easily should do the trick. A transformer of course works too, but doesn't pass phantom power without some tricks. They might have taken either approach.
Note that such devices are dependent upon level, so they won't do as much on a dynamic mic as a condenser on the same source.
Yeah, from one of the reviews linked they say it's a pair of transformers.
It works just as well to stick a transformer on the output of a preamp where it's easy to saturate and you don't have to worry about phantom power, and it doesn't cost AUD149.