The Signal app never did the "primary device serves message contents to companion devices" like WhatsApp did. Ever since Signal introduced the desktop client,

The Signal app never did the "primary device serves message contents to companion devices" like WhatsApp did. Ever since Signal introduced the desktop client, its multidevice functionality has involved fully fledged signal-protocol sessions between each device. It sounds like WhatsApp is now adding something like this to their client, though it sounds like some of the details of the scheme differ. In principal the signal protocol works fine for a multidevice scenario.