Signal knows who you are taking to. You can build a network of contacts based on that information. When you send messages your phone number is protected but your ip address is not, and the receivers phone number is not protected. So you can find two people chatting based on that information. The app automatically sends a delivery receipt when a message is received to the other user, exposing the senders phone number and IP address.
However, opposition in the country is backed by western agencies and NGOs, and likely their primary means of communication is signal since it's backed by western intelligence, meaning, western actors believe it to be safe from external interference.
I'm not arguing that the west is reading messages. I'm arguing that they believe it's a safe haven for their agents because they pay money to ensure it's safe for their agents. If it wasn't, they wouldn't use it. Its the same reason why the intelligence community in the west is a large supporter of the tor network. They use it in the field and operate their own exit nodes to protect their operations.