Not necessary, because whether people agree or disagree isn’t relevant to the reasoning. The author’s argument is based on the fact that sometimes people think a poem expresses contradictory ideas; people might agree or disagree about this, but it’s still the case that some readers will think the poem expresses contradictory ideas.

Not necessary, because although the author thinks writers of great poems do not intend contradictory ideas, that doesn’t imply that those writers can’t intend multiple, non-contradictory ideas. Maybe the writer can intend three or four major ideas, as long as they don’t contradict each other.

Not necessary, because whether people agree or disagree isn’t relevant to the reasoning. The author’s argument is based on the fact that sometimes people think a poem expresses contradictory ideas; people might agree or disagree about this, but it’s still the case that some readers will think the poem expresses contradictory ideas.

Not necessary, because the argument is based on the fact that sometimes readers think a poem expresses contradictory ideas. These contradictions are not intended by the author. So the argument is based on believing a poem expresses something that’s not intended; whether readers can discern what the author DID intend is not relevant.

Necessary, because if it were not true — if an idea might NOT be part of the meaning of a poem, even if a reader thinks the poem expresses that idea — then the premise concerning what a reader believes a poem expresses no longer supports the conclusion. The fact a reader thinks a poem expresses contradictory ideas would no longer suggest anything about the meaning of a poem.