Luckily, the DMA has a heap of requirements around what their messaging interoperability will have do. For one thing, it will enforce the providers to not downgrade any encryption along the way, so FB etc will have to handle messages without them being decrypted first. There are some great videos that the matrix foundation put on their YouTube channel of talks that go over much of this.
You can use any matrix client you want with it. The closed source one is just making bridging more straightward and adding some little quality of life features.