I think that what Takahashi was trying to say was that it is not optional - until you reach a certain point, as Sagawa did. It makes sense, elementary school children learn certain drills - but that doesn't mean that they use those drills forever.
It's clear from what he was saying that it was an enunciated part of the system - note that it appears on the densho, and that he refers to a series of breath exercises taught systematically. Also, we know that other people who trained at Sagawa Dojo have brought back breathing exercises from Sagawa directly. And of course, there is Tokimune, who also spoke about and taught breathing exercises.
I was referring more to the realization that using breath to activate ki is optional. It seems as though this is a realization that Sagawa arrived at himself.