You received a perhaps overly-precise but correct response. If you are indeed on a single ticket, something which you should confirm for yourself and not rely on AP or the Japanese carrier to tell you, it is the responsibility of the late-delivering carrier to rebook you should you misconnect.

If, on the other hand, AC delivers you on schedule and you fail to present yourself for your onward flight as required by the onward carrier, that is generally-speaking your responsibility.

You may confirm that you are on a single ticket by looking at your e-ticket receipt (not simply your confirmation) and looking for the ticket number. There should be one ticket number for both segments or if there is a ticket number shown for each segment, the two numbers ought to be exactly the same.