Or maybe something like… the existing IETF standard for IM: XMPP. With plenty of implementations, support across the globe, servers that can scale up to millions of sessions.
I hope they go for Matrix, like France did, but I get a feeling they'll be hit hard by NIH.
One relatively low effort path for a government might be to simply adopt something like Chatsecure and fix it.
I'm not saying that Matrix is bad, or that XMPP is definitely better, but the facts are there and we'd better stick to that than hold on how we feel about each piece of technology.