I probably should have used the words "presentism" and "eternalism" instead of "A-theory" and "B-theory". I've added a footnote which I hope clarifies this.

The point is that these are theories about what exists, and not just theories about how we should talk about time. To say that all of time is even "there" to be coordinatized is, in essence, to sign onto the B-theory as opposed to the A-theory.