For one-to-one comms Signal is easier to get started with. Matrix requires a hosting service (or just reliance on the main matrix.org instance), or trust in a server administrator to run one. But it is good software. But unfortunately for group chat it's not as easy to get started as Discord, due to the encrypted nature of Matrix. But Discord shouldn't be considered secure or private, not really. It's just more proprietary spyware, at the end of the day.
Certainly, no matter what, no SMS.
Can you post a source? Because I haven't heard such a thing.