This is true, but for people like me, beside the point. A car is either open or closed, to the extent the terms mean anything. A computer doesn't have to choose! It can be closed, until you press the "Yes I Want To Possibly Break My Box" button and all it has to do is stop checking executable signatures and let things that aren't from the official source run. (Isn't this how Android works? I don't have one.)
This is not a fundamental tradeoff that Apple with some regret has been forced to make to make their product easy to use. Gruber is hiding his arguments behind a technical tradeoff that is entirely manufactured by Apple in the first place.
A few weeks ago I bought my wife an HP Mini running Windows 7. She will never crack the hood. It is an easy to use machine, browser here, open office there (which, by the way, I was allowed to install), IE locked away behind this icon over there to access her ActiveX-based time-and-attendance system. It is one application install away from having an "app store"; for instance, Steam could probably be installed in about five minutes.
And I could turn it into a Linux machine in about half-an-hour. Because none of that ease-of-use actually requires the machine to be tinkerproof.