People usually judge others based on what they can see ("you are what you eat", but not only). Why not based on what tool they use? It contributes to establish a persona.
A Time interview from August 1997 confirms that:
http://allaboutstevejobs.com/sayings/stevejobsinterviews/tim...
Also, in fairness, the circa 1997 Thinkpad was the best business machine on the market. Period. Macs had to catch up. The Macbook wasn't even a figment of anyone's imagination back then. The Powerbook 3400 (c. early 1997) was faster than the Thinkpad, but the productivity suite available at the time wasn't anything to write home about, let alone use for serious presentations.
All that is to say, I'm not as shocked on reading this as I guess I should be. From everything I've heard, Jobs only took dogfooding so far. If he didn't believe an internal product was the best for a given purpose, he didn't dogfood it. (I would be curious to know if he used Mail, for instance...)