It's a challenging book to read; don't be surprised if some of Melville's writing never quite makes sense; not everything has to make sense. The narrator, who introduces themself at the outset, is a person of varying reliability - you'll need to decide how much.
Reality edges closer to Melville's fantasy.
And the crossing of Elephant Island by foot - a task never before and -very- seldom accomplished since.
While both relatively short accounts towards the end of the book, either would merit top-10 lists of remarkable achievements.
He didn't get to Antarctica on that voyage, much less achieve the expedition goals. But in truth he achieved an expedition that has resonated so much more.
Regarding the journey across Elephant Island, that too was incredible. I note that there are tour companies providing opportunities to do this, though I haven't managed to do it yet.