The argument is pretty straightforward actually: the iPhone SDK is Apple's intellectual property and they can charge whatever they want. The fact that developers have to pay for Macs and iPhones to make apps and users have to buy iPhones (all of which Apple makes a margin on) is irrelevant. Copyright doesn't have a "but if you're already making money off of it you can't charge people other ways" clause. Disney makes money off of you when you watch their movie in theaters, stream them at home, buy their branded merch, and go to their theme parks. Nobody calls the royalties/licensing fees that Disney makes off of the above items a "Fleece", even though you arguably need to buy one of them to enjoy the other (would you really buy their merch if you don't watch their shows?), or that their IPs would be less valuable if one of the revenue streams didn't exist.
All of this is orthogonal to the question of what apple is doing is "fair" or whether is good for consumers/developers/apple, and I wish discussions on whatever Apple is doing wouldn't conflate what they're entitled to do vs what they should do. In other words, rather than arguing "The technology fee is a fleece, they shouldn't be charging the fee when they're already making money off developer machines and iPhones", argue "Apple is entitled to make money off their SDKs, but it'll be better for consumers/developers/apple/everyone if their SDKs were freely available". But I suspect that's less persuasive. The DMA throws a further wrench into this, because it limits what Apple can do with their IP, but given that the article doesn't try to reference any DMA provisions to support his argument, I don't think the author is interested in this line of argument.