This is a great question, and the distinction can be confusing. Multiple sentences may appear in print for different reasons.
First, they may be presented as examples for study, such as in a grammar book. These sentences illustrate the same grammatical point, but they are not intended to be read as a continuous passage.
Second, a reading passage may contain back‑to‑back paragraphs that each consist of a single sentence.
The guideline regarding multiple sentences applies to the first situation—sentences presented for study. It should not be applied to a reading passage simply because that passage includes several single‑sentence paragraphs.
When a reading passage is displayed, it should be formatted just as any passage beginning at the normal margin, except that the entire passage is shifted to the right.