For a poet he threw a very accurate milk bottle.
The remark is a dry, comic observation built on the contrast between literary identity and physical ability. Hemingway treats being a poet as though it made accuracy in throwing a milk bottle surprising. The humour depends on that deliberately unreasonable assumption, turning an aggressive incident into a memorable description of character.
In A Moveable Feast, Hemingway is referring to the poet Ralph Cheever Dunning, who threw milk bottles at him during an unusual encounter in Paris. Rather than offering a serious judgement of Dunning’s poetry, Hemingway ends the anecdote with an understated assessment of his aim. The line is characteristic of the memoir’s literary portraiture, in which writers are presented not only through their work but through eccentric habits, arguments and everyday incidents. More broadly, the remark works as a humorous way of acknowledging an unexpected competence that does not fit someone’s assumed identity.