Can a content entity type have a bundle which is a configuration entity type as with the contact form?
This may be easier to understand with Nodes, since Nodes have been used by almost every Drupal site builder.

Nodes are a Content Entity. Nodes have bundles (aka content types). For example, a blog. Now, to define the blog type, a new Config Entity is created - the Node Type entity. Config entities have keys. The key for the blog Node type is 'blog'. This key identifies the node type. But the blog bundle is still a Node entity, and therefore is a Content Entity.

The contact form works the same way.

So to answer your question, no, bundles of Content entities cannot be config entities, as bundles are just a subset of the original entity, which is a Content entity. But a Config entity is used to define the bundle (allowing the types to be exported as configuration).

You could almost compare it to the 'type' field on a Content Entity being an Entity Reference to a Config entity that defines the type.