I think the point of the post is that good designers should be generalists. A good designer would read (or re-read) the iOS HIG in order to have a good idea of the platform's guidelines before beginning work.
"There is nothing special about the web that makes it more or less challenging than designing on an iPad. They are just different."
Differences are exactly what makes something special. Specialists are those people who have in-depth knowledge of the differences and know how to make use of them ... and know how to make use of those differences now, not after they experiment and figure things out from first principles.
So advertising for an OSX designer, or an Android designer is a sound decision. I've run a team of Solaris sysadmins and yes I could have hired Linux sysadmins (unix is unix as the cliche goes) ... but I won't - Linux admins may know the fundamentals of unix, but I want people who really understand Solaris and how to get the most out of it. And I don't want to wait whilst a Linux sysadmin learns the differences between them.
To be a little critical of the author, it doesn't sound like he has much experience as a designer - the blog post sounds like something written by someone who doesn't know what he doesn't know. Isn't the saying that the 'devil is in the details' (or differences) ... I would have thought that a good designer would understand that and never argue that all you need is generalist knowledge.
Now you're comparing system administration to design - this doesn't really work. Technical skills like this don't translate well, and I never claimed they do.
The post is about UI and interaction design and those principles are the same across all platforms. Design patterns for a specific device, operating system, or medium (e.g. the browser) are not something you need to be a specialist to learn, analyze, or pick up - you need to understand the fundamentals of design.
Early iPhone designers who built great apps had spent much of their careers designing on the web. They had to learn and come up with new design patterns, but since they were good, it was well within their skill set.
Had they? My view of the iOS pioneers was that they mostly cut their teeth designing and developing for the Mac.
Heheh I knew I should never have used an analogy, there is always something that won't quite hold true to the point you are trying to make :) The main point I'm trying to make is I believe in any field, the details are always important and things like patterns are shortcuts to speed up a process, but they aren't a fix-all for whatever problem you are trying to solve.
If you compare the best iphone apps today to the best iphone apps from the start, I think most people would argue that today's apps are far slicker and easier to use. Why is that? it's because people have built on previous experience of what worked and what didn't. Experience matters, understanding the differences matters, specialist knowledge matters and it's why advertising for a specific type of designer matters.
I will agree with you that a good designer can, with enough time, achieve the same results as a 'specialist' designer - but that's the whole reason people advertise for a specific skillset, they don't want to wait.