I think it is fine to link the concepts of miai and ko threats, because many cases of miai applied in the context of life and death actually implies ko threats for the other player. To put this relationship explicitly, suppose two points are miai for the life for a white group. However, the above supposition is typically stated when white must get one of these two points in order to live, i.e., miai is a condition for life. If miai is a condition for life, then it follows that either of these two points is a ko threat for black.

Therefore, I find it useful to draw a relationship between these two concepts, provided that the context where "miai" is used is sufficiently scoped. In general, the concepts of miai and ko threats aren't equivalent, and neither is a subset of the other. It is obvious that not all ko threats are involved in miai positions. On the other hand, the term "miai", when used in contexts such as endgame, aren't typically associated with ko-threats. For example, we can consider two unrelated points that are worth two points each as miai, but we don't consider these points as ko threats for a five-point ko.

Just my two cents. =)