I don't buy it either, this number is way too high, if it was true, this forum would be full of Bitcoin millionaires. Most likely they just used the logic that 1 address = 1 person, and then took addresses with more than $1mil, but it's obviously deeply flawed, as one person can and usually does own multiple addresses. And then there are exchanges which hold enormous amounts of coins, and many of them are in cold storage so it's even harder to determine how many coins people own.