I wonder if Apple is genuinely having issues or they just have internal rejection targets per genre or have some criteria tied to expected sales.

The size of target demographics of music creation apps are basically zero. Even if the music created were to be sold on Apple Music to be played million times, that won't get reasoned and traced back to be attributed to these apps. These are likely a "waste of disk that nobody buys and won't lead to MAS user retention", in a narrow view concerning just the Mac App Store.

App Store review process was always known to be inconsistent, unfair, unpredictable, just wrong, a reviewer gacha gambling of its own, etc. etc., but, I mean, causes of rejections mentioned in the article sounds a bit bogus, like bad fault tree analysis.