Thanks to everyone who replied.
I have talked to a lot of people on the Fediverse who think DMs appear in public feeds (local, federated, search, etc.).
This is not the case.
However, it's so easy to create a DM and accidentally set it to public that we see examples of the mistake all the time. If you don't check the UI carefully, you might think they're really DMs.
Anyway, I'm strong disagree.