The Signal service’s Terms of Service do not prohibit non-official clients, so anyone can build and run a Signal client from source.
The Signal devs claim they don’t want people distributing forks of the foss client that point at the official Signal servers, but they actually release it under a free software license, so you are free to do precisely that, and all they can do is complain.
The software is free software governed by the software license, and the API/service is governed by the ToS. Neither prevents alternative clients.