according to the website you mentioned. these are just estimates. even the website is also saying there is no way to find out actual number of bitcoin users. everyone can just make estimates.
we can only know the number of wallets have bitcoin. knowing how many individual have bitcoin is impossible. because some people have multiple wallets while some people keep their bitcoins in exchanges which are held by one master wallet.
the website above do gives some interesting facts tho. it was fun reading that.