I'm not as familiar with Sam Harris, but the impression that I have is that he's treating and conveying mindfulness and meditation as a science that is measurable and comparable from person to person, which I do not think it is. It is "scientific" (to these people) in the sense that someone is trying to extract the essence of something that is subjective and attempting to quantify it or find reason from it, when in fact, there is no more to be found than the original subject matter and/or it is baseless with reality or what is well-understood. I think many have tried to substitute it with religion as if it is better and grounded in science, when it about the same in quality in many regards.
Definitions and names are tricky things. The names we give to things the first time around usually will encompass multiple concepts. As we try to separate each of these concepts, we try to assign new names but in discussion (in the case it's in good faith) conflate them with the older names of things because not everyone can stay current to the most recent names, and they may understand the concept you are trying to isolate in conjunction with something they consider it a part of. Relativity used to be general relativity, and now can also refer to special relativity, efficient in some ways and confusing in others.
Then there's the question of how much can be objectified and reproduced, and whether that effort is worthwhile compared to embracing a concept in its entirety with its truths and lies because it is more realistic compared to what can be built from what you managed to discover and build from exclusively. We use Newtonian Mechanics despite its inaccuracies instead of quantum mechanics for designing buildings because it is close enough at the scale of what we intend to solve. I consider mindfulness, meditation, and in some cases religion to be similar, in the sense that they seem to approach you at the scale of being a person or at the scale of your humanity full of subjectivity in need of a subjective solution or explanation, instead of a machine to be worked on piece by piece. No matter how much objectivity is possible, individualized application is going to have subjectivity or uncertainty. How far to go one way or the other, and how relevant that would actually be to the situation at hand, is always questionable.