I don't think people do treat market share as a proxy for profit share. You don't really need a proxy for profit share because it's easy to measure. And if market share is publicly avilable information, so is probably profit share. It does make some sense to treat market share as a proxy for market power, though -- business people do this all the time.

An alternative interpretation of the article is simply that Apple is targeting a more profitable segment of the market -- smartphones.

It happens to be the case that this segment has grown really fast and have both the highest margins and the highest total profits. This is perhaps not the case in most industries, which may be interesting.