So, from the comments here, it seems that people are under the impression that mastering engineers work off of the individual instrument tracks (the stems). This is not true. The article itself explains that a mixing engineer takes the individual tracks and mixes them. The final mixdown is then sent to the mastering engineer. It's called "mastering" because this stage of processing happens on a single track - the master track. Buying music in a pre-mastered form would still be a single lossless file, which is no different from Bandcamp allowing you to download the final lossless mastered version. The pre-mastered tracks will just sound worse and have wildly different volume levels depending on how they're normalized.

If you want the actual stems of the song, that would actually be the music in it's pre-mixed form. It wouldn't even be mixed at that point, let alone mastered.