That isn't anything special. Every smartphone can send SMS over wifi. You need to connect to the in-flight wifi to use the service. The router on the plane has a hole in the firewall that allows unpaid users to access the specific HTTP server to send and receive SMS messages. Absolutely nothing special about it.

Eventually this would be no different: Starlink can provide (firewalled) data service to iPhones to support messaging apps.

Speculation is that initially there would be a small number of satellites periodically passing overhead, leading to the 30 minute send/receive times, and likely not supporting non-SMS messaging apps well.