Even if you argued this text is biased in favor of Burton, I think Speke would have been a incredibly poisonous person to know. 

Additionally, if Sidi Mubarak Bombay is the true center of this story as is hinted at — the summary of his life, apart from Burton and Speke, should have earned a chapter of its own, as they received. And not tucked into the epilogue. Arguably, he did go further and see more then either, so the content was there.