All Bitcoin transactions are public, in the sense that everyone can see which addresses sent and received the coins. (However, this includes no information about who owns the addresses.) This is an essential part of the Bitcoin protocol: you can verify the validity of a transaction that sends coins to you, by tracing the "chain of custody" of those coins all the way back to their creation, and checking cryptographically that each transaction along the way was properly authorized.

So since transaction data is public, everyone can see the balance of every address. Thus it's possible to determine that 927 addresses own over half of all coins in existence. This doesn't tell us who the human owners are! It's quite likely that some of those addresses belong to the same people, meaning that wealth is actually concentrated in even fewer hands. On the other hand, it's also possible that some of those addresses are shared property; for instance, some could belong to a mutual fund.