I'm a little confused, although it's not your fault. There used to be a cohort in between Generation X and Millennials called Generation Y, and they were significantly different than either of the generations on the border. For reasons that were never clear to me, the Powers That Be in marketing starting pretending like they didn't exist, put some of them as the youngest newly minted Gen Xers and the rest as an older half of Millennials that aren't really anything like the rest of Millennials. (Although they also split "Millennials" into Y.1 and Y.2 to represent this obvious and substantive difference; which is stupid as all get-out since using the older labels of Gen Y and Millennials already accomplished that task quite well.) It sounds like what you're describing are "disappeared" Generation Y, and I would suspect that that's a much bigger cohort than Generation Z, for various reasons, including, as you point out, the fact that they have a lot of disposable income. And as I said, the graph lists supposed players, not customers; and I question where they got data from 8 year olds in the first place. But whatever.
I didn't look very carefully at the chart until just now, and it's got major problems. As you say, Gen Z is mislabeled; a bunch of Alphas are included, making it look larger than it really is. Also, Millennials, especially if you consider both generations within the Millennial "generation" are in fact significantly larger. And Gen Xers and older literally aren't even graphed, as if we somehow don't even exist.
This isn't a true pie chart, it's just some kind of interpretive graphic for data that is clearly egregiously wrong. I know this is starting to sound a little conspiracy theoryish, but this Narrative that Gen Z is this big cohort, but oh, we're actually going to count Gen Z wrong, just not label Millennials and hope you don't notice it, and somehow "lose" the data entirely for any players who are Gen X, Generation Jones or any Baby Boomers who may still be playing (I'm sure there's some) and hope you don't notice that we're pretending nobody over 45 plays D&D at all.
Suddenly the disconnect between why WotC is saying that they're targeting Gen Z for their products, but acting like they're targeting Gen Y and Gen X at least as much so starts to make more sense. There's a whole cohort of players that they're literally not even showing us and not talking about. I know in C suite lingo, targeting the younger crowd is sexier than targeting people who are established in their careers and preferences but who have much of the disposable income is what management wants to hear, but fudging the data on who your player base actually is, which it is more and more apparent that they are doing, is starting to look pretty shady.