An intuitive interface is one that is easy to learn - but it still requires learning. That people already know how to use it is orthogonal to how easy it is to learn to use.
While there are certainly many factors that go into determining the number of users who know how to use a device, orthogonal is a poor word for describing the relationship between people who know how to use the device and ease of learning. The two are definitely correlated, IMO.
Consider the extremes. Something that was impossible to learn would have zero users that know how to use it. Everyone would know how to use a device that required no learning.
It's this latter hypothetical to which the iPod/iPhone/iPad aspire. So why should Apple care to make the point that so many people already know how to use it?
After some thought (and I may be slow to the conclusion that everyone else has come to already), I think it is because they knew that "It's just a big iPod Touch" would be one of the main objections. This particular point about the 75MM users was an attempt to turn an aspect of the device that could be perceived as a big negative into a postive.
For me - the relatively uninitiated into the world of pitchmen - it's an interesting piece of marketing.
