I can only imagine why there are no comments or discussion items for this question.

Question says "the percentage of sales along some dimension"

Answer explination: "cannot be aggregated along any dimension in the measure group that contains the measure"

So how is a percentage aggregated across some demension considered Nonadditive?

A semiadditive measure can be aggregated along some, but not all, dimensions that are included in the measure group that contains the measure.

The question described a semiadditive measure.

Percentages can be Nonadditive, but if they are made across some deminsion they are semiadditive measure.