I bet the majority of people who say you can't code on the rMB don't even know how to code.
Developing a heavy app for iOS may be difficult, but coding itself shouldn't be impossible. i.e. Writing in HTML is coding (not to be mistaken with programming), and it's totally doable.
It all depends on your environment. No doubt some find cutting code in XCode with multiple simulators and IB open might be a little constrained (due to screen size more than anything), but actually writing the code should be more than fine. If you're a full-time iOS dev and an ultra-portable laptop is your only machine you're probably a little crazy anyway.
Personally: I've already written a couple of solid libraries for Go on my rMB and enjoy it. The screen is great and at 1536x960 there's more than enough real estate to have multiple files open at once, or half-docs/half-editor. Just like any other modern laptop.
Indeed it can depend on the situation and complexity of what you're coding. But I successfully created an HTML5 and CSS3 website entirely on my spare 12" iBook G4 that I keep at my parents house. I used TextWrangler and TenFourFox for the most part. And a little Photoshop CS2 for the images. It really makes you appreciate a screen that's higher resolution than 1024x768!
Of course running simulators and live preview/code editors would demand more workspace but even a 13" MBA at 1440x900 is pretty usable. Or a scaled resolution on a 13" rMBP like 1680x1050. People can always make do.
Well it really depends on what do you mean by "coding".
I work with XCODE all the time and I can tell you that is really hard to code on my 11'' MBA.
I also work a lot with HTML and is not convenient to have the page and the inspector on a small display.
I do exactly like the guy in the video, I use a portable machine and I connect it to an external display. No way I can work on a 11 or 12'' display all day long, let's be serious. Maybe I could work on a 15'' display, but even a 13 is not enough to work for 10 hours straight.
When I worked on embedded system I was able to use a 12'' display without plugging an external display, it was coding but in a different environment. But at the end of the day I missed the bigger screen.
The point is that macbook is a great portable machine but if you need to code for many hours you find yourself comfortable with a big screen, a keyboard and a mouse.