It is not 100% private. Admins can read your messages if they choose to investigate your messages.
It will not get blasted out to the whole fediverse; just to the recipient you indicated. (Unless an admin from the previous point reads your message and publishes it publicly on the fediverse)
You do not get to do anything naughty with it; expect to be caught if you break the rules.
Unless I run a Lemmy instance myself (which is possible), I have zero reason to trust an instance's admins.
Even if my instance's admin happens to be the founder of privacyguides.org, that doesn't mean he will never read any "private" messages (or be forced by someone else to hand them over).
What definition of safe are you using which makes a private messaging system without privacy safe? What would have to occur for it to become unsafe, if not being private does not make it unsafe in your eyes?
They absolutely do. A private messaging system which is not private is the definition of unsafe. Especially in the context of a post on !privacy@lemmy.ml