Sounds like tosh. How an extensor muscle being stronger can help you flex harder I have no idea, and as far as I know, science doesn't know either.
In this months rock and ice, Doctor Julian Saunders says:
The assertion that some ephemeral imbalance of agonist-antagonist strength leads to elbow and bicep tendonitis, carpal tunnel syndrom, and various injuries to the hand tendons and tendon pulleys is absolute bollocks. Even after you apply the construct that this is kung fu magic using rice, and therefore Eastern medicine, and therefore we Westerners may have trouble comprehending, it is still way beyond the pale.
Choose your expert.