In my view, the act does not mention Diwali or festival bonuses. It only states that if some festival bonus is paid, then that amount is allowed to be adjusted against the bonus payable as per the Payment of Bonus Act. Bonus is only payable if the salary is less than Rs 2500 per month. Perhaps this amount was recently increased to Rs 3500 per month—I am not entirely sure.
In other words, organizations do not have a legal obligation to pay bonuses to highly-paid employees. If they do pay, it is considered an ex gratia payment and not a bonus under the Payment of Bonus Act.