This "category" was made to essentially not count the Windows Tablet PCs. Read the article, they framed the market so that the iPad would fit and a few other Android based tablets from Archos and the Nokia Maemo stuff.
So that is why Apple got 87% market share, because they cut out all the other players basically.
2011 will be the real test, when the players like RIM, Dell, HP get into the market with Android 3.0 and other offerings that are competitive, we'll see if Apple is able to maintain their iron grip. As it stands, Archos and Nokia were not promoting their offerings at all, compared to Apple that was marketing the crap out of the iPad.