It's a matter of how much you want to get into it, both in practice and in terms of the discussion here. If we just want to look at how silly the WTS position is that goes without saying. We do know it has spiritual roots, but the fact is most teach what might be called "fast food yoga" by some (often called "power" yoga) and couldn't even really guide you in the spiritual stuff even if you were interested, even if they'd like to read you some spiritual philosophy and create that general atmosphere in the studio.
Generally speaking something like yoga follows the model of physical to energetic to spiritual, so it's just on a continuum of going toward a subtler and less tangible perception of the body, but it's still the body. Just because you're not thinking, and you naturally do this all the time during the day anyway, it doesn't mean your mind goes away. What the concern is of course is that you forget the beliefs the WTS teach, or you even stop holding on to them as tightly. Without getting into it in much detail which people are not likely to be interested in, there can be feelings of bliss (rather than just loose and relaxed) even when people don't even really know what they're doing, and you can imagine that's going to be preferrable to the kind of feelings they're used to having as a result of indoctrination and social pressure. Because it's a matter of working with the body people are also empowered as individuals as opposed to being dependent on being in a group, and that of course doesn't go over well either.