The privacy loss is "phone number has registered a signal account"

It does not

  - conclude the user has or even has a signal account
  - who that person is talking to 
  - what that person is talking about
  - when those texts or messages are sent or received

What can you infer here that becomes a security risk? I guess if signal is outlawed before you have installed or your number was ever associated with an account? But it still have plausible deniability there