"계산서는 제가 들고 있어요."
Translation:I am holding the bill.
Yes, but the purpose of this exercise is to point out that 은/는 are not specifically designed "to mark" the subject in a sentence.
Those topic tags 은/는 can be used to categorize the complement (whether direct object or others) as well. In this case, the direct object.
계산서는 제가 들고 있어요 = 계산서를은 제가 들고 있어요
I would think so, since typically pronouns like "I" or "you" get put at the beginning of the sentences... Not 100% sure, but it could be based off of small, differing indications (like emphasizing "I" am holding it rather than someone else, or I am holding "the bill" rather than something to else. Kinda like the subject/object-marking particles indicate). If a fluent Korean speaker could chime in to clear up that would be super helpful!