When I said 8, yes I meant both the 8 and 8 Plus.
My original argument: "It's known that the 8 outsells the X in total units". When I interpret best selling, I interpret it in units, not ASP/revenue. What does best selling mean to you? Highest revenue or most units sold? Like you said, you can infer from sales numbers. This is what makes Tim Cook's statement very ambiguous, and you don't even realize it.
If you look at Statista, there are graphs correlating on unit sales for iPhone 8/8+ vs X, and they support that the iPhone X is the highest revenue driving model as of the last few quarters, but the 8/8+ have sold more units.