Unable to speak to why gp said so, but I have a few reasons why.
In the mid to late 1800s and early 1900s philosophers, writers, and poets were all talking about how "this newly formed country" needed its own Epic Poet that embodied the spirit of the country, ala Homer, Milton, Basho, Virgil, Dante, Kālidāsa, et al. This pursuit is also where the phrase "the great American novel" comes from.
The philosopher Ralph Waldo Emerson wrote an essay on the subject; defining what qualities, to him, such a poet would require.
This essay, The Poet, inspired Whitman to write his epic poem Song of Myself, with such qualities, that would, in later editions, become his collection of poems Leaves of Grass.
Emerson read the first edition and wrote Whitman a letter all but proclaiming him the poet he was imagining, noting "I find it the most extraordinary piece of wit & wisdom that America has yet contributed."
It became a point of contention between the two when Whitman published this personal letter as the foreword of the second edition.
In the mid to late 1800s and early 1900s philosophers, writers, and poets were all talking about how "this newly formed country" needed its own Epic Poet that embodied the spirit of the country, ala Homer, Milton, Basho, Virgil, Dante, Kālidāsa, et al. This pursuit is also where the phrase "the great American novel" comes from.
The philosopher Ralph Waldo Emerson wrote an essay on the subject; defining what qualities, to him, such a poet would require.
This essay, The Poet, inspired Whitman to write his epic poem Song of Myself, with such qualities, that would, in later editions, become his collection of poems Leaves of Grass.
Emerson read the first edition and wrote Whitman a letter all but proclaiming him the poet he was imagining, noting "I find it the most extraordinary piece of wit & wisdom that America has yet contributed."
It became a point of contention between the two when Whitman published this personal letter as the foreword of the second edition.
https://www.poetryfoundation.org/articles/69389/from-the-poe...
https://whitmanarchive.org/item/loc.02109
Personally, I still think Leaves of Grass is "the most extraordinary piece of wit & wisdom that America has yet contributed".
It still has opportunity for improvement, but as yet, the most extraordinary.