Well, it was simple relative to other smart phones. Not simple relative to a phone phone, sure.
So maybe Gruber should have said that UI is simpler, but not the iPhone. If iPhone is so simple, why aren't other big companies as successful at creating new phones? Clearly, things are not as simple as some people would make you believe.
Also, a jump from Razr or Palm (for example) to iPhone is much bigger than that of iPhone 1.0 to 3.0.
Had Apple released a Razor-like iPhone and then iterated to iPhone 1.0 system over a year or two, I'd buy Gruber's argument. But iPhone was clearly 'from the future' when Apple presented it and everyone else was scrambling to catch up. So Gruber's argument is not really serious.
Other companies aren't as successful at making new phones because they're reluctant to abandon everything they have to create new systems and models. Their minds are stuck in a single pattern. Nobody making those mock-up sketches for the iPhone thought that Apple would get rid of a numberpad. Nobody. Every indie designer making a mock phone kept the numpad there. You can't necessarily throw lots of people at a concept and expect it to be better. It takes a certain moral bravery to get rid of everything and start anew.