Just to make it clear: Matrix could be said to fall back to SMS, or IRC, or Slack, or whatever other bridges folks chose to run on the network. In reality, it's not so much a fallback as Matrix just providing glue between the various existing networks. And if you want E2E, you would use clients which support it, in a room where it is enabled.
However, I'm wondering if the point of this blog post was more to try to encourage Moxie into supporting 'unofficial' clients on the Signal network, or enabling federation - rather than look to other solutions. Opening up the Signal network would of course be fantastic, not least in terms of letting clients or federated servers like Matrix bridges provide interoperability to the rest of the world - and especially to E2E capable clients!
Re: Yes, you can pick all three, and you can do it right now.
However, I'm wondering if the point of this blog post was more to try to encourage Moxie into supporting 'unofficial' clients on the Signal network, or enabling federation - rather than look to other solutions. Opening up the Signal network would of course be fantastic, not least in terms of letting clients or federated servers like Matrix bridges provide interoperability to the rest of the world - and especially to E2E capable clients!