The couple in the above tale seem to have got a promotional \marketing discount from the builder\vendor. This is not a real discount at all in my opinion.
This is to be distinguished from the situation where somebody goes to a builder and says "If I buy x number of units what discount will you give". In the UK, this tactic does seem to yield a real discount in some situations. I do know a few people who have grouped together in an informal way to buy UK properties in this manner. Good discounts (7% - 10%) have been achieved. Unfortunately, I have no experience of dealing with any companies who offer a service in this area, but I rather imagine that it would be difficult to trust the advice which they give: after all, they need to flog units which they have pre-booked, so it is hard to see how they have the poor punter's interests at heart.