> If BMI is unaffected, wouldn't that mean muscle loss as well?

For a given person, BMI is equivalent to weight (because height is fixed). If fat went down, but weight/BMI stayed the same, that would indicate an increase in the non-fat components of body weight.

Nope BMI is just a "weight to height" ratio ( https://www.nhlbi.nih.gov/health/educational/lose_wt/bmitool... )

This can lead to some funny errors in edge cases, like Mr.Universe Arnold being morbidly obese according to BMI

BMI is purely a weight/height calculation. If BMI is unchanged that just means total weight is unchanged — meaning, if fat was lost, muscle must have been gained.

my reading is no change in mass but less fat, meaning gained muscle instead of muscle loss...

That's what I was thinking when I read it, but it seems strange that it isn't explicitly mentioned.

It also seems quite surprising to me that drinking pomegranate vinegar would increase muscle. Makes me wonder if there's something I'm missing.

Which doesn’t really make sense, as drinking ACV alone will definitely not result in a body recomp/the addition of muscle (every meat head lifter, myself included, would be drinking a few bottles a day in that case).

It could have changed their height...

... I'll see myself out.