In the poetic illustration, a person is asked about his condition, and the reply states the condition without repeating the subject. The question has already identified the person concerned.
قال لي كيف أنت قلت عليل
He asked me, “How are you?” I answered, “Ill.”
The predicate «عَلِيلٌ» is understood as part of «أَنَا عَلِيلٌ»: the speaker is the suppressed subject, identified by the exchange itself. Uttered in a different situation, the bare adjective could leave the intended referent unclear.
The implied subject must therefore be reconstructed from the available indication. Grammatical possibilities alone do not authorize replacing the contextually intended «أنا» with an unrelated «هو».