Huh???
You put down a down payment before you secure a mortgage???
You must have some strange practices there in Hong Kong.
How aboutthe fact that Hong Kong Banks had a lot of money wrapped up in Europe and the USA???
You seem to think that all markets work in isolation.
The safest way is of course do it as you say, check with the bank first about how much you are likely to get for a mortgage. But local people have a rough idea and many if not most go ahead with the property deal first.
The Mainlanders in this case were either very confident as well thinking they should qualify for a loan, or they were looking to resell the flat during the long (10 months?) completion period. However as no local banks will offer them a loan and they could not resell, their deals fell through.