well, granted, strictly speaking, once the flat goes into the secondary market, there's nothing that distinct this flat from others that were sold original as a private apartment.
but in the first place, the term 'discount' from hdb is also quite a misnomer. the pricing of first hand hdb flat is quite market driven, and like said the pricing difference from the secondary market is not much different.
if you want to distinguish secondary and primary purchase of a hdb flat, then the 80% percentage of people staying in a flat purchased from hdb would probably be much much lower, I'd even guess in the tune of maybe just 40~50%, as most people have already sold their original hdb purchased flat and switched around at market prices.