any operation involving HBD could trigger interest payment!

That's overly broad, IMO :-) Consider a hypothetical operation that changes the time period for which funds are staked. Such an operation would have no reason to call adjust_balance(). On the whole, I think that the best description is still that operations that change the balance are the ones that are intended to trigger interest payments. It's really only a current implementation detail that results in calls to this function even when the balance isn't changed (ie. when the delta parameter value passed is 0). And it's easy to imagine a future where the call immediately returns after checking that value, before triggering an interest computation.