In the term "Subject Matter Expert" (SME), what does the qualifier "Subject Matter" mean? Are there experts without subject matter (other than perhaps Irwin Corey, the "World's Foremost Authority")
From the page you reference:
In general, the term is used when developing materials (a book, an examination, a manual, etc.) about a topic, and expertise on the topic is needed by the personnel developing the material.
The reason you might call someone a "subject-matter expert" is to clarify that the person is an expert on this subject matter. A medical doctor can certainly be called an expert, but if he's being quoted in a book about plants, then he's probably not a subject-matter expert.