I think the poster's point is that they're not. One has apparently been well designed to be able to scale to new features, and the other one appears not to have been.
Linus has often pointed out the huge pain that the fragmentation of the ecosystem causes that is often uncritically identified as a choice.
Yes, converging towards standards is almost always a pain point for someone, but the ecosystem overall benefits from having much less ground to cover. And honestly, objectively almost any of the large mature DE's seems to be feature rich enough to use as a starting point.
Maybe KDE was a little too flexible for my tastes, but it sure worked better (most of the time - not always).
I also remember thinking - why the heck is gnome the default?
Haven't used linux as a DE for a few years now though, so no idea what the current status is...
For example, many distros only really differ from their parent in theming, replicated but differently named applications and so forth, which could all be offered in one distribution with less workload, but the same degree of genuine choice.
Same is true for DE's I think. Many of the things that differentiates DE's could probably be offered as configuration within one DE, with less workload but no or very little reduction in diversity.
If you want to get flamed to death, mention the word "configuration" on a Gnome mailing list.
There are people who think that things could just be configuration options, and then there are people who insist that the default configuration should be right, and that too much configuration is detrimental to the user experience (because it's hard/complex/time-consuming) and to development time (if you have two sets of two configuration options, that means you have four configuration to tests, and so on). It's not like these folks don't have a point, either.
Perhaps if this DE is KDE. Gnome devs aren't interested in a configurable DE.
Yet another DE means extra set of conformance tests, bugs to fix, look-and-feels, supporting pages. These all mean money but your customers may not even care about this feature.
So let's say GNU/Linux can use whatever DE and it will always be. But RHEL is just a business product derived from it, with a selected "blessed" packages. There're no conflicts.