It may or may not be more secure, but it is privately owned and always official business. The employer can take legal action for improper disclosure of messages from that system. It also means other segments of government can never use these messages in any kind of legal proceeding without a prior valid search warrant.

If you are a "politically sensitive" activist group trying to make an actual difference (vs. performative and feel-good stuff) - then you should assume that the spooks are watching, if not actively infiltrating, and plan accordingly.

(If your goals are mostly performative and feel-good - then having the FBI spying on your group is A List bragging rights and cred, no? Congratulations!)