There were TikToks advertising Mixy that made it look way better than it is. It's just a not so good stem separator, like AI. The quality of both songs is really bad, it only lets you do short sections of songs, and there's no way to edit levels if the vocals or beat is too quiet.
On the other hand the TikToks I watched never had problems, seemingly every song had magical perfect quality and went together perfectly. In the actual app this is almost never the case. You will get mixes where it doesn't even get the right audio from one of the songs, so one track is basically silent, the quality is atrocious, or it doesn't even try to sync them up in terms of bpm or section. Like I said, when I would watch videos of Mixy on TikTok before it was released, it magically seemed able to do all of these things.