From what I understand in this thread, being exclusively an Xbox One owner means I can't purchase 360 games, not even backwards compatible games. Is that correct?
You can buy 360 games through the Xbox One store, or the online 360 store, but if you don't have a 360 sitting around the biggest limitation is that you cannot use your Microsoft Account balance to buy those 360 games. You instead need to use a credit card or PayPal or similar.