iPhone X wasn't as supply-constrained as previous iPhones. Whatever that says about iPhone X demand, it won't make a revenue difference at this juncture.
Apple's improving their operations every year, so they can ship more units now than they could last year or the year before. Previously, if Apple was completely supply-constrained between launch and the holiday season, they would only earn revenue on the quantity of iPhones they could ship. Now that Apple has improved their supply chain volume, their launch quarter units shipped will go up purely as a function of that, and their revenue will also increase as a function of the higher unit price of iPhone X.
The only difference, if there truly is one, will come in the next quarter or two. If a product is supply-constrained, there's a backlog of demand for it that only gets met down the road. If the product isn't supply-constrained, there isn't as much backlog and consequently, unit sales and revenue won't be as high. But that doesn't affect the launch quarter because the launch quarter is completely limited by supply.
If iPhone X was a total flop, of course, that would have shown up by now. But "iPhone X less popular at launch than iPhone 6" is a hypothesis we can't judge yet.
Edit: It's also really, really easy to tell whether a product is supply constrained. Just go to apple.com and see how long it'll take to ship. See if people are scalping them on Craigslist for a premium over the retail price. These kinds of things happen with major console launches and used to happen with Apple's product launches, but Apple is one of the biggest consumer electronics brands in the world, has some of the best operations in the world, and contracts with the best high-volume electronics manufacturer in the world, so it's more than possible for them to overcome the supply constraints that "plagued" previous launches.
iPhone has an installed base of 1.3 billion users. That's more than 1/6 of the world population. At this point, there probably aren't even enough people in the world who can afford smartphones for demand to grow as fast as supply. If Apple's operations and supply chain can grow faster than the global middle class, they're going to get past their supply constraints, and apparently they already have. That's not necessarily a reflection on this year's iPhone.