I guess it depends on the definition: I'd say owning Bitcoin means owning owning your keys. Considering there are only 53,218,057 funded Bitcoin addresses (based on yesterday's data), and considering many Bitcoin owners have more than one funded address, there are far less than 50 million Bitcoin owners.
On the other hand, if I use a broker to buy shares of a certain company, I can say I own shares of that company. So I guess (by now) it's okay to say you own Bitcoin when you "have" some on an exchange or ETF. By that definition, 1 in 6 or 7 sounds plausible.