Sounds like the British media is playing a game of let's-you-and-her-fight. The Sunday Times apparently has written a pig-ignorant piece about the fantasy genre and Rowling's place in it. Pratchett responds, as one of the genre's "made" men, to set the record straight. He's not writing so much to attack Rowling as to speak up for his colleagues (he needs no defense himself, as he belongs to the canon already). But then the British media paint it as some sort of churlish and unprompted attack on Rowling.
In 1977, I'll bet People magazine was full of articles about how George Lucas had reinvented Science Fiction, reinvigorating a staid and stodgy genre, blah, blah, blah. And I'm sure that if Asimov ever said, "nu, that's not quite right," the blogosphere would have said, "Sour grapes! Say it ain't so, Ike!"