yes, it's 50,000 miles for business class...but it's NOT a bonus of 50,000...instead, they give you extra so that your total Asia Miles earned from the trip is 50,000 (including the base miles + class-of-service bonus)...e.g. if you fly HKG-LAX roundtrip in business class, normally you would get 18,120 miles (including class-of-service 25% bonus), now they give you a bonus of 31,880 miles (i.e. 18,120 + 31,880 = 50,000)...
the outbound flight (HKG-USA) needs to have happened on or before April 30...while the return (USA-HKG) can be as late as May 31...all bonus miles will be credited after the promotion period...