Oh, I hate trying to get into the minds of others, but I'm going to belly up to the mental bar anyway. With all the dangers inherent.
I think that, Joly-like, Deiss has a Theory*. (He has more than one Theory, actually. But there's one in which I'm interested.) He is very insistent on the importance of expressing site over cpage, feeling it's the Right, Traditional, and One True Path for Alsace. I think that he makes wines with that intentionist chip on his shoulder, so to speak; they must be exemplars of his argument, as much or perhaps even more than they must be wines.
To pursue his Theory, of course, he must have blended-variety sites. Because his Theory doesn't work very well if he only has, say, pinot gris on the Burg. But the problems in practice, if not in Theory, are obvious. The traditional Alsatian grapes don't ripen at the same time. They don't express the same levels of success within a given vintage. So to make a successful wine, some sort of leveling, averaging decision must be made.
Deiss regularly makes that decision in favor of ripeness. Which is one thing with riesling or pinot blanc, and something very different with pinot gris or gewurztraminer. To avoid the harsh acidity of underripe riesling or pinot blanc, to get it ripe, the fatter grapes (which are usually harvested earlier than riesling, unless they're destined for vendanges tardives) just hang there. Losing acidity. Getting sweeter. Changing the tannin/juice ratio in favor of the former. And so forth.
I mean, look at the color of a Deiss blend. It's dark. Really dark. That color isn't coming from riesling, it's coming from very late-harvested darker-skinned grapes like pinot gris and, to a lesser extent, gewurztraminer. Producers who let similar grapes hang longer than the average don't produce wines of that color; you won't see truly ros-hued pinot gris at Zind-Humbrecht very often, for example. But you'll see decidedly blush wines from Deiss, even with all the light-skinned grapes in the mix. That's beyond suggestive. There's also botrytis to consider, and the potential reddening effects of pinot noir and chasselas ros, but even when those aren't a factor the wines are still darker than most.
I suppose that Deiss has convinced himself that these wines are in balance. Whether he has or he hasn't, I think that what most satisfies him about them is their impact; impact that is not achievable without pushing the grapes as far as possible, impact that gets them noticed, impact that makes them stand out, impact that forces one to consider them and thus to listen to Deiss' Theory**. After all, gewurztraminer stands out just fine on its own, but start cutting it with riesling, pinot blanc, and whatever else, and it starts losing impact. So if it's essential to grab people's attention via their palate, you've got to get that back, and one way is to dial the gewuztraminer or pinot gris element well past eleven to something like 23.
Almost everyone in Alsace makes a blend (for one must count crmant and pinot blanc/auxerrois), but most of those are blended later in the process. Quite a few wineries in Alsace make Deiss-style blends, however, and nearly all of them faced with the same harvest decisions that confront Deiss. With the very occasional exception here or there, I have never seen blends as heavy, dense, and dark as what Deiss produces.
As far as I'm aware, his vinification is pretty traditional and standard. So I guess the conclusion is that it all happens in the vineyard, and for the same reason that [insert the name of your least favorite Central Coast pinot noir producer here] thinks pinot noir isn't ripe until it has to be watered back to 14.5%: it's the taste he wants. Moreover, it's the taste he needs to make his Point.
Me, I think he mostly obliterates terroir. Which is a shame, because I've had wines from the domaine, albeit in the increasingly distant past, that expressed terroir very well.
*I've got a theory: it could be witches. Some evil witches...which is ridiculous 'cuz witches they were persecuted Wicca good and love the earth and women power and I'll be over here.
**All brontosauruses are thin at one end, much, much thicker in the middle, and then thin again at the far end.