It was in the list just buried in all the other rules....

Ensure the meshes involved do not have very small faces and edges. Booleans operations do not work on meshes that have zero area (or very small) faces or zero length (or very short) edges.

One of the reasons booleans get a bad rap is that there are a lot of rules and so it's easy to break one or more of them and then spend a lot of time scratching your head trying to discover the problem.