They are referring to message metadata.

Even if they don't show the content of messages, if they can show that phone number A is sending messages and getting replies to number B then that's all the government needs.

https://signal.org/legal/

For the purpose of operating our Services, you agree to our data practices as described in our Privacy Policy, as well as the transfer of your encrypted information and metadata to the United States and other countries where we have or use facilities, service providers or partners.

They store metadata, which is distinct from encrypted data.

Are you saying sealed sender is a lie?

https://signal.org/blog/sealed-sender/

When you send a traditional piece of physical mail, the outside of the package typically includes the address of both the sender and the recipient. The same basic components are present in a Signal message. The service can’t “see into” the encrypted package contents, but it uses the information written on the outside of the package to facilitate asynchronous message delivery between users.

They have a list of encrypted messages, who it's from and who it's to, based upon the sealed sender description. If you are using phone numbers then you are not anonymous, and a TLA agency can search known bad numbers even if Signal does not try to build that graph.