If West Point has requested a medical exam for you, does that mean you have a good chance at getting in? I read somewhere that they only request it if they think you are a competitive candidate.
All appointed candidates will be qualified by DoDMERB or be granted a medical waiver but not all qualified candidates will be offered appointment. It is similar to being academically qualified. I will say that if you did not appear to be competitive they might not put you in for a medical at all.
Requesting a medical exam is not a major indicator of likelihood to receive an appointment. It is certainly an important step, but generally just completing a number of administrative steps along the way will unlock a medical exam opportunity. Getting qualified medically is a much bigger deal. The pool of candidates who medically qualify is considerably smaller than the pool of candidates who have exams scheduled.