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

If something claims to be unsafe and delivers, that doesn’t make it any more safe.

Safe and private are synonyms in this context.

It is not unsafe.
It is not 100% private. Admins can read your messages if they choose to investigate your messages.

These points contradict one another.

I don’t know any of the technical details, but my understanding is they’re very unsafe. Our instance has a warning recommending that you take conversations involving sensitive information to Matrix