On re-reading the sort article, I realize it only says they will be compelled to share "encrypted messages" and "information to support investigations". It never says anything about decrypting the messages. If that is literally all it is, then it is quite misleading.
What use would ciphertext be to GCHQ? I’m fairly certain that “encrypted messages” describes which messages they will have to provide in plaintext, not the format they will come in.
Your interpretation is likely right. But it's interesting to consider the other. Maybe they can get metadata from the ciphertext, such as size. Or maybe they are just interested in other metadata such as time. Or maybe they have some cryptographic attacks, maybe involving extracting a shared key from the peer's phone, or key injection via sim spoofing.
