The article misses a BIG issue; banks will not finance developments unless a certain percentage is pre-sold.  The threshold for financing is generally 65% pre-sales.  

If you're a developer who needs financing to build a highrise, do you think you'd prefer to have third party buyers making much of your profit by buying in advance and re-selling the purchase contract?  No.  The developer would rather get it built and sell the units at their market value once complete.  Because banks add this requirement, the developers have no choice but to look at it as a cost of financing.  

As to the suggestion that someone is avoiding property transfer tax by re-selling their right to buy, their is no merit to that.  No real estate has changed hands.  The only thing the pre-sale buyer sells is the right to buy, not the actual real estate.  

As to whether the ultimate sale attracts the 15% foreign buyer tax, that is a function of the residency of the ultimate buyer once the building is complete.