Sorry but I did'nt understand your comment :
this site gives you ALL the existing BTC adress because the total amount is really the right one (16.5M BTC...) so you have all ways of BTC storage...
that is how bitcoin works. blockchain is a public ledger that contains all the transactions that has ever happened. if you know how you can simply write a code to get all the information such as how much bitcoin belongs to what address.
and that is what that link in OP is showing.
what you can't tell is that who does these addresses belong to. for example many of these addresses containing a large sum of bitcoin belong to services such as exchanges.