SMS and iChat may have the same interface, but they are not mixed in a single client. In fact, Apple does not provide an IM client. The central complaint in this article is that the same message was being communicated over email and SMS, without the sender being able to distinguish between the two in any way. I did not read this article as complaining that the interfaces for SMS and email were too similar; I read it is complaining that they were in the same client. The only complaint about the interface I saw was that it grouped all messages together, independent of the subject.

[Edited to clarify which part of the parent I was responding to]