Earlier quoted context omitted.
Of course it's realistic. The Malagasy people are descendants of Polynesians who crossed the Indian Ocean to Madagascar, in canoes. With calm seas, there's no reason you couldn't make such a trip. Canoes can have sails too, and you can row your way out of the doldrums.
There's a big difference between coastwise travel in the Indian ocean and crossing the middle of the south atlantic. Even the big portuguese ships couldn't round the cape of good hope til what - the mid 1500's?. Also it's ~5200 miles from Madagascar to South America.
Hawaii, it should be noted, is a clear contender for "most remote place on the planet", given the lack of even small islands outside of the Hawaiian island channels for thousands of miles.