True, but the term "aiki" in and of itself had been in use for hundreds of years prior to the "coining" of the term "aikido." Teachers in the Daito-ryu Jujutsu world (like Sokaku Takeda sensei), for example, use the term "aiki" very frequently and not as the usual aikido definition of "blending with ki" kind of thing.
Richard Harnack wrote:
Ki is the central character of Ai Ki Do, it cannot be excluded. Neither is it preeminent.
This can become a very slippery slope of linguistic history of terms. Like many of the terms used in the martial arts, "Aiki", "Ki", etc. have come to be used in both overly specific and general ways. Your example of Sagawa's comment on "Aiki" being one which seems to refer to a particular class of techniques sharing a particular quality.
The not so simple matter is how the individual using the term understood it and what importance they assigned to it. The person who said it first does not necessarily say it best.
One of my favorite anecdotes about how tunnel visioned martial artists can become (especially, in Aikido) relates to "O' Sensei". It seems an American Aikidoka was relating to a Japanese something about O'Sensei. The Japanese stopped him and asked which "O' Sensei" was he talking about because in Japan this term is used to refer to certain persons who are perceived as master teachers. His point being, that as an accountant, he had an "O'Sensei" in accounting who meant a lot to him.
Doing linguistic history and tracing the development of a concept is great fun and very enlightening. It can also become a completely futile exercise because having explored all the possible parameters of meaning of any given term, one ultimately comes back to the meaning one gained early in training. What is added is experience. What is lost is the freshness and excitement of the first moment you achieved some understanding of the concept.
I think that we need to keep distinctions clear when speaking of terms (Aiki, etc) in the technical sense and in the broader philosophical sense.