The whole point was that he went far enough away (sailing, Australia) to not only speak with the anger of the injustice (what happened to him), but to target the structural aspects (how enslavers are made) and how the justification was built in to the Constitution. In so doing, he anticipated the evolution of structuralism, post-structuralism, and critical theory.
It seems to have been just one historian who found and validated the story. The author sister was the subject of fawning but incomplete treatment (her story was sympathetic, not damning); the historian found the author's work and traced the author's sailor gigs to validate the connections.
There can be an element of passivation to history: "don't worry, we're understanding more and getting better". Thanks to both the author and the historian, it appears much was understood even then, and we shouldn't rest on tides that might not be rising.
Critical theory has fatal flaws and anger almost never leads to insight or the right incentives, but how else would normalized and profitable injustice be stripped of a legal veil?