True, the frequency response is fairly flat, but the iPhone's amp still has distortion, which does add some sort of colouration (frequency response isn't the whole story).
With your logic, are you saying that everyone who has a FiiO L3 or L9, or similar LOD connectors just wasted their money on a cable that has doesn't do anything beneficial?
If you'd read Uncle Erik's response again, he basically just said what I said... in a different way: you could still overload the second amp. Or in other word, if the second amp is not able to cope with the level of the first amp, then it could still distort. But if the second amp can cope with the first amp's level, then it should be fine. And... we're talking about an iPhone here. iPhone doesn't have the most powerful amp section, so it most likely won't be able to deliver that much power. If in doubt, you can just use the volume control to drop the volume on the iPhone.
You'd be right, though, if we were talking about double-amping with something like, say... an O2 as the first amp and the E12 as the second amp, and the O2's volume pot was pushed to max on high gain. I admit double-amping is bad in some cases, but it's not bad in all cases.
And no, I'm not saying that they wasted their money. A direct line-out (LOD stands for Line-Out Dock in case you haven't realized) is still better most of the time, as it's less likely to overload the second amp, and you don't have to fiddle with the volume control on the device.
Hm, okay I'll trust what you're saying. I do notice a difference in sound between the double-amping and single amping though, particularly with the soundstage and instrument separation.