I think that's exactly wrong. Macs are already the trucks in his earlier analogy. Apple only sells 5% of computers, but they're the highest-priced 5%, so they make an outsized portion of the market's revenues on them.
The iPad doesn't disrupt high-end notebooks and desktops one bit. It disrupts the low-end, the $600 Dell and HPs that make up the bulk of sales by volume (but which are really little more than loss leaders for the manufacturers).
Even if tablets get to, say, 75% of the market, it will mostly disrupt cheap PCs. If most of those tablets are iPads (debatable, but certainly not impossible) Apple will make a hell of a lot of money.
Apple would seem to think this way as well. Otherwise they wouldn't introduce a $500 product that disrupted their $2,000 products, especially when they think that nobody else will make a compelling tablet if they don't.