Well, they couldn't: Slocum disappeared in 1909, and Buller, I'm pretty sure, wasn't even born then. (I can't find biographical details online, but he's a modern illustrator and cartoonist whose career seems to stretch back to about the turn of the century - this century - so he's likely in his forties.)
The original Sailing Alone Around the World was a non-fiction book by Slocum, loosely based on letters he wrote during, yes, a three-year period when he sailed 46,000 miles alone in his sloop Spray from Massachusetts to Massachusetts, the hard way. It was published in 1899, and is one of the standard classics of the adventure-travel genre, on that short shelf with books like The Worst Journey in the World and The Long Walk.
Buller has published a long list of books for children, along with his wife Susan Schade - she writes and he draws mostly early readers and middle-grade books. The two of them did a comics project a few years back, the intriguingly-titled Nudism Comes to Connecticut. But this one seems to be all Buller, working from Slocum's original book.
His version of Sailing Alone Around the World translates Slocum's book into comics form, with mostly big square panels with cleanly-ruled borders and large sections of Slocum's text as captions. His art has a lot of cross-hatching, and his people are slightly cartoony in a vaguely Edward Koren style - both of which both reflect his story-book experience and work well for a story set over a hundred years ago.
Joshua Slocum was a seasoned seaman in the early 1890s, in his mid-forties with years of experience as a master of sailing ships around the world. He'd run off to sea (the time it finally took; he'd tried before) at the age of sixteen in 1860, and had been working since then. But, with the rise of steam ships, his skills were less and less valuable, and it looks like he had a period of unemployment.
So he decided to refurbish a small sloop, the Spray, and plan for that round-the world trip. Buller starts by adapting the work Slocum did to refurbish and rebuild Spray, which took two years - that gets us through the first twenty pages of the book, and then Slocum sets off on his trip.
His initial plan is to go east, and he gets to Gibraltar without too much incident. But there a friendly British ship tells him the southern coast of the Mediterranean is still full of pirates, making Slocum's intended path to the Suez Canal much too dangerous.
So Slocum instead went the other way, recrossing the Atlantic at its narrowest point back over to Brazil and going down the coast of South America and through the Straits of Magellan, where bad weather delayed and diverted him for more than a month.
But he did make it through, and got across the broad Pacific to Australia. He spent some time there, meandering up the east coast and across the north, before setting on across the Indian Ocean, stopping in South Africa and then finally heading back home across the Atlantic a third time.
Along the way, he saw a few pirates - mostly at a distance, mostly in small boats, and mostly "natives" - put in at Juan Fernandez to honor the memory of Alexander Selkirk (the original of Robinson Crusoe) and ran into a lot of seamen he knew from his earlier career or that knew him by reputation. He also ran aground at least once, and had to do extensive repairs in harbor (various ones, all along his journey) multiple times - par for the course for a wooden sailing ship on a long, rough trip. For a book about sailing alone, there's a fair bit of convivial dinners and 19th century pleasantries, including dialogue that I assume is straight out of Slocum's book.
Sailing Alone is the kind of book that in the reading feels like it was much easier than the work was in reality, so the reader starts to think "I could do this! That would be grand!" in his best 19th century diction. But it was an impressive accomplishment, and Slocum got a fair bit of fame from doing it. His book was a bestseller, making him enough money to buy a farm on Martha's Vineyard, where, of course, he ended up not spending much time, since he preferred to be at sea. It was on the Spray that Slocum disappeared, about a decade later - the assumption is that he died in a rough sea, but, like Ambrose Bierce, his body was never found, so readers can make up any pleasing stories they want.
