I don't see why they would need to interface with iMessage at all. Perhaps I'm missing something, but it just seems like they're responding to text messages sent to a particular number.
It also makes scaling their side of the system easier. Instead of having some sort of CRM/SMS gateway system, they can just hook however many Macs they want to to their account, and all messages sent to it will get copied to all of them. This makes it easy to hand off a communication session between agents in a way that's transparent to the users of the service.
Of course, the downside is that it's an Apple-only protocol, so they will be prematurely excluding users of non-Apple devices, but maybe that's a tradeoff that the company's willing to make at this time.
It also says "iMessage" at the top of the conversation.