The issue isn't somebody proving you have a Signal account, the issue is somebody identifying members in a specific group used to communicate about a protest. That way a state actor can find the phone number of a key organizer, persuade their phone provider to share their location, and take them out.

Here's a description of how it worked in Telegram before they added a setting to disable it last year: https://medium.com/adamant-im/telegrams-anonymity-hole-how-t...