That’s only for targeting. From what I understand ELITE does not include device compromise or eavesdropping. If feds want to compromise a device that has Signal, they would use something like Pegasus that uses exploits to deliver a spyware package, likely through SMS, Whatsapp, or spear phishing URL. (I don’t actually know which software is currently in use but it would be similar to Pegasus.)

As mentioned by someone else, they just need to take the phone of a demonstrator to access their signal groups.

https://freedom.press/digisec/blog/new-leaks-on-police-phone...