The billing API tells you about resources that have charged you, whereas I think the parent comment is talking about resources that are either “active” in some sense where they will charge you at some point in the future for the usage you’re accruing (but where this usage isn’t metered in a way visible to the billing API until it “commits” somehow); or, worse, resources which are “exposed” in some sense where they could charge you if they received a request, but happen to not have received any yet. (These resources are very easy to lose track of.)

Another way to think of this question is “what is my current exploitable attack surface, against the ‘attack’ of causing AWS to charge me lots of money?”