Hell, I assume this is only popping up after seeing Matrix eat their lunch and being chosen over them for the German Armed Forces, French government, and German healthcare system.
Threema and Matrix are quite different. Matrix is a centralized / federated system where data is stored on the server. Threema uses a decentralized protocol, where data and metadata (including groups) are stored on the devices only. This has advantages and disadvantages, depending on your requirements.