So the activity streams vocabulary says a 'Link' object has properties href, rel, mediaType, name, hreflang, height, width, and preview and nothing more; a 'Mention' object extends the Link object without adding properties, and example 58 shows how you construct a Mention object which has the property 'summary'... that is allowed nowhere in the same document for that specific message.

No, the spec is absolutely not ambiguous.