Good point. I did watch the keynote. SOS and Find My coordinates are the only explicitly specified use cases. But it's a paid service with the first 2 years free. I made the assumption you should be able to send text messages, because asking people to pay for a service that only lets you communicate with emergency services and that's all seems like a much less compelling product which is a bit un-Apple like. But then again, they have done lousy products before so it's possible they went with the less-compelling option.

Per a video someone else posted it seems there's communication with (probably just) emergency services which (for some use cases) is probably more useful than just a personal locator beacon.