"The database key was never intended to be a secret. At-rest encryption is not something that Signal Desktop is currently trying to provide or has ever claimed to provide," responded the Signal employee.
Tl:dr Signal is 'insecure' in the same way your mail is 'insecure' after you've opened it and it's sitting on your counter. What, you don't keep you mailed locked up inside you own house just in case someone breaks in?