Rupert Brooke (1887-1915) was an English poet known both for his talent and his good looks. He wrote idealistic sonnets about war, but never saw combat. Though he sailed with the Royal Navy to Gallipoli in 1915, Brooke died from sepsis in Greece before they arrived. He was 27. He was buried on the island of Skyros.
Emily Dickinson (1830-1886) was an American poet who lived a quiet and reclusive life, but wrote extensively. Only a few of her poems were published in her lifetime, and they were highly edited to fit the conventions of the time. Not until 1955 did her work appear as it was originally written, and the true extent of her talent become apparent.
In comparing the two, D.B. is suggesting that the poet who wrote idealistically of war but never experienced it did not grasp its horrors, while the more thoughtful recluse (living in the time of the American Civil War) did.