Exactly.
In English I’d say: intraword emphasis can only be opened and closed in the same word.
In the language of the spec, I think it might be something along the lines of:
A single * can close emphasis started by a * that is both right and left flanked iff that delimiter run was itself opened by a * that was both right and left flanked.
Or maybe along the lines of point 9
Emphasis begins with a delimiter that can open emphasis and ends with a delimiter that can close emphasis, and that uses the same character (_ or *) as the opening delimiter. There must be a nonempty sequence of inlines between the open delimiter and the closing delimiter; these form the contents of the emphasis inline.
added: if emphasis was opened by a right and left flanked delimiter, no non-word characters (whitespace, punctuation) can occur in between.
The difficulty of phrasing this in the language of the spec may imply that it’s technically difficult (or that I suck at writing spec 
)