Earlier quoted context omitted.
Your comment exposed me to the big gap between the meanings of muckraker in US and UK English: 1. (US) One who investigates and exposes issues of corruption that often violate widely held values; e.g. one who exposes political corruption or the poor conditions in prisons. 2. (Britain) A sensationalist, scandal-mongering journalist, one who is not driven by any social principles.
I'm American and I've only heard a muckraker described as number 2.
As someone said, it was a pejorative term that was reclaimed, and that makes a lot of sense to me now.