They're not saying it's not secure for normal conversation, but not up to the national security standards for such coversations. It not being a proper tool for the job is what makes it "unsecure".
Yes, this. I was about to post my own comment and will instead reply and upvote yours.
This strikes me as setting the conversation to be whether it's 'secure', and can then everyone can discuss that part - instead of the fact that's not where or how that conversation should have been happening at all.
