I don't know where you are getting information about global sales, or why you are pulling this into the discussion, since this article was about US sales only, which is the basis for my points. As for "saturation" I don't think this is much of an issue. The Windows PC market is plenty large, sufficiently large such that Apple doesn't need to steal much of it away every quarter to continue to grow their share. For the last few quarters, Apple has been garnering more the half of the total unit growth in the US.
The article quotes that Apple has failed to break into the worldwide top 5. I'm explaining why this is.
I have absolutely no disagreement with the fact that Apple are increasing their share of a saturated market at Windows expense, however my points are:
1) This expansion will inevitably hit a ceiling, and
2) The same logic doesn't apply to global sales where the market isn't saturated.