It doesn't contradict it. He talks about Apple, you talk about Apple's customers. Thinking of it from the company's position, other's monopolies are bad for you, not necessarily your monopolistic advantages.
Yes, and as for developers, they're developing for a market (i.e. Apple isn't supplying materials, just opportunity). It's sort of like how Detroit was developing for the petroleum market. The idea there, though, is that in developing for a market, you should have general domain abilities that can span markets. In other words, if you're an Apple developer, you depend on the Apple market to sell your product, but you should be able to take your development ability and develop for Windows or Linux should the Apple market collapse.
Obviously, it's not guaranteed that you'll be that smart. Detroit should have been smart enough to take their expertise in developing for the petroleum market and transfer it to success in the alternative energy market. But it's still not as bad as having a key component of your product be supplied by a monopoly (just ask your local jeweler).
I see what you're saying, but hasn't nearly every Apple customer effectively done that? A datacentre could maybe rip out Xserves and replace them with equivalent 1U boxes running some generic Unix apps (e.g. Tomcat) but anyone who has developed on the Mac platform is beholden to Apple.
Not that this is necessarily a bad thing, and not that there is an economic way to stay completely platform agnostic, but it does contradict what you say.