"The simple definition without reference to turn or CBM is not sufficient.
In the absence of any pre-conceptions, a person asked to stand with their feet slightly apart, then to place their RF in front and in line with their LF will most likely turn the body slightly to the left. "
I would guess, at least if they had any exposure to dance habits at all, they would be much more likely to turn their body to the right.
However the question itself is based on an unlikely premise: leaving aside the tango and a few unusual situations in the others, CBMP positions don't tend to come from nowhere, and they are not generally executed with the body square to the feet. Instead they are generally preceded by a same-side lead on the preceding step, which may be simply carried over without change to become an opposite side lead on the CBMP step.
In other words, CBMP features the result of body turn relative to the feet, but it's turn that happened sometime previously.
If this setup is satisfied, then the simple instruction to step across the body is likely to result in something approaching the desired result, at least to the degree to which the beginner is physically capable of finding comfort in a CBMP position.
"They will have met the requirements of the simple definition but are not, and have not stepped, in CBMP."
Nor in your example will they have stepped across their body as I had suggested we instruct them. I specifically did not choose to phrase it relative to the standing foot, because while the book is written that way, it's not how most people are used to thinking - instead of foot-relative, we tend to be body-relative. And while CBMP is defined with respect to the feet, the reasons for using it are mostly with regard to the body, or more exactly the fitting together of two bodies.