If I'm expected to believe a messaging app is secure, the first thing I want is an open protocol. An open source client would be nice too, but honestly I'm fine with just the protocol.
I do not need to have had a hand in developing any of this. It's not my expertise and, like you, I'd feel more comfortable having it developed by the experts.
I would trust iMessage about 95% less if I had written, or even implemented, the protocols myself, and I consider myself a pretty good developer.