The thing about textual evidence is that you can't cite an entire text (obviously). You have to selectively choose what to quote in order to support your claims. Additionally, people can write one thing, and then write other contradictory things. Or they can act in ways that contradict what they write. It is from this totality of evidence that non-quantitative methods draw their conclusions. To get to the point, I'm not necessarily claiming that Nancy Maclean (the historian "caught cutting sentences in half") is in the right here, but if you actually follow the debate it seems quite nuanced and the internet critic hadn't actually even read most of the book they were criticizing (and also clearly has certain political leanings to boot). Certainly nothing like "throwing away half the data that didn't support my findings."
The JEL review which I quote in the linked blog certainly had read the book, and called it "replete with significantly flawed arguments, misplaced citations, and dubious conjectures". And if cutting sentences in half, to remove something which directly contradicts your thesis, doesn't count as historical malpractice, then what would?
It's just not that simple. I'm not making a value judgment on the book (I haven't read it), but a person can say two things in the same sentence and the broader context can make it clear that they're just covering their ass, for example. Perhaps that's not what's going on here. Perhaps the book does constitute "malpractice." But...I think the situation is more complex than you're giving it credit for, and I wouldn't be comfortable drawing conclusions without a greater familiarity with the book and the responses to it. I also don't give a lot of credence to the blog you linked, since they use (as one of their two pieces of evidence) a critique which openly admits it hasn't actually read the thing that is being critiqued.
To your last point, plagiarism, for example, definitely counts as malpractice and humanities professors lose their jobs for it.
