He's claiming it's an over the counter substance.
Why would the NBA ban anything that's over the counter? What if he just went to GNC and bought something to put on a few pounds of muscle? That's illegal? That doesn't make sense to me.
Quite a few over the counter supplements (ie stuff you can buy at GNC) contain substances deemed illegal by the NBA and the NFL drug policies. Most of them contain steroid precursors or masking agents (the star caps in Minnesota). They are legal for consumers to buy and own but the sports leagues have banned a substance in them. Several NFL players a year test positive for something they blame on supplements. Most of those substances can be serious performance enhancers if you take them in the right dosages.
Bottom line is that if you are a professional athelete, don't take any supplement or vitamin that your team trainer doesn't give you directly.