It says right on the home page that they use Windows Media Lossless as the encoder. A 'lossless' encoder is akin to Zip for compressing other types of files (although the algorithm is different and takes into account characteristics of audio). No data is lost - only the file size is smaller.
That is in contrast to 'lossy' encoders such as mp3, wma, aac, etc that use 'perceptual coding'. A lossy encoder throws away data that its model deems inaudible, so not only is the file smaller, but the data is different than the original.
A file that has been compressed using a lossless encoder will sound identical to the source from which it was encoded.