Signal and whatsapp is developed from same developer. They only thing they have same is encryption protocol and nothing more. If users of whatsapp need to contact with signal users they need to agree with certain toc and rebuilt the app. Practically, no signal user will wish to communicate with whatsapp users bcz of lacking encryption and data privacy issues. I dont believe this third party chatting idea of whatsapp won’t work bcz signal and other popular messengers like matrix,tg is for privacy and security. Also i don’t think that no platforms will support this idea of whatsapp as chrome proposed FLOC.