BMI uses an exponent of 2, for physical reasons it needs to be around that value, but the "correct" value might be different; it appears that GP believes it should be higher.
The point about body builders is that BMI as a metric assumes a fixed muscular profile, so doing (an healthy amount of) muscle training can wildly affect your BMI independently of your fatness.
If is still likely a good measure at a population level as very few people that have BMI > 30 are also fit. But you need to contextualize it to your own case.
