All this time I've been assuming that Apple keeps iMessage as iOS/Mac only as an anticompetitive tactic, but that this would never be admitted in writing and they'd be smart enough to only talk about verbally in unrecorded meetings.

These emails actually make me change my mind on both aspects. First, apparently they were at one point undisciplined enough to talk about risky topics in writing. Second, the reasoning in those emails is actually very reasonable. iMessage was a non-revenue-generating service that cost them money and was funded by sales of Apple devices, so it's being offered as a service to users of Apple products. This isn't unfair behavior on Apple's part.