You're right, regarding Mastodon. I won't edit my other comment, though, both to preserve the original chain of thought and because that brings up another discussion.

To quote the EFF:

We feel that the intended usage of the feature will not determine people’s expectation of privacy while using it.

Offering people a feature with preexisting expectations, similar to other things that fulfill those expectations, then telling people "We know it looks like a duck but don't expect it to quack!"

...It begs the question: was the feature really a good idea?

That's right; they've always been documented to be DMs, not PMs.

But because of the discordbabies people confuse both.