It doesn't look like that's possible! You cannot expose a Content Fragment (CF) via Content As A Service (CAAS) directly, you need to use a custom Content Fragment component as a proxy. This is because CAAS uses the Sling Model Exporter functionality and a Sling Model can only be binded to a component with a "sling:resourceType" property. Unfortunately Content Fragment Models in the DAM do not have that property and so a Page (normal or Experience Fragment page) is required so that you can add your custom Content Fragment component on it that then in turn links to your Content Fragment Model in the DAM. Your Sling Model then binds to the custom Content Fragment component rather than the Content Fragment Model itself.
TBH, it's all a bit fake... Adobe are just making a big deal of Sling Model Exporter and marketing it as a CAAS / headless CMS feature.