The discussion distinguishes restricting a subject to a property from restricting a property to a subject. These differ in both direction and the alternatives ruled out.
Compare “Zayd is only a poet” with “Only Zayd is the poet here.” The former restricts the asserted characterization of Zayd; the latter restricts the relevant bearer of the characterization to Zayd.
When interpreting a Qur’anic or poetic example, identify which alternatives are being excluded and which relation is affirmed; otherwise the qasr can inadvertently be reversed.