Based on that summary, I'd say that it's interesting because it draws on enough real science to be superficially plausible, while appealing to enough emotional triggers to make people want to believe in it enough that they'll be ready to ignore any inconsistencies.

Superficially plausible: Individuals being selfish and pursuing their own interest above that of others is arguably the main source of suffering among humans, and you can easily generalize the argument to the biosphere as a whole. Superorganisms are indeed quite successful due to their ability to suppress individualism, as are multi-celled creatures in general. Humans do seem to have a number of adaptations that make them more successful by reducing individualistic tendencies, and it seems plausible to claim that even larger superorganisms with more effective such adaptations could become the dominant power on Earth. If one thinks that there is a general trend of more sophisticated superorganisms being more successful and powerful, then the claim that "evolution is not directionless" also starts to sound plausible. The "humans have a chance to help with that process but aren't intended to see the outcome" is also plausible in this context, since a true intelligent superorganism would probably be very different from humanity.

"Evolution leads to more complex/intelligent creatures and humans are on top of the hierarchy" is an existing and widely believed meme that similarly created a narrative that put humans on top of the existing order, and this draws on that older meme in two ways: it feels plausible and appealing for many of the same reasons why the older meme was plausible, and anyone who already believed in the old meme will be more inclined to see this as a natural extension of the old one.

Emotional triggers: It constructs a powerful narrative of progress that places humans at the top of the current order, while also appealing to values related to altruism and sacrificing oneself for a greater whole, and providing a way to believe that things are purposeful and generally evolving towards the better.

The notion of a vast superorganism that will one day surpass and replace humanity also has the features of vastness and incomprehensibility, two features which Keltner and Haidt claim form the heart of prototypical cases of awe:

Vastness refers to anything that is experienced as being much larger than the self, or the self's ordinary level of experience or frame of reference. Vastness is often a matter of simply physical size, but it can also involve social size such as fame, authority, or prestige. Signs of vastness such as loud sounds or shaking ground, and symbolic markers of vast size such as a lavish office can also trigger the sense that one is in the presence of something vast. In most cases vastness and power are highly correlated, so we could have chosen to focus on power, but we have chosen the more perceptually oriented term "vastness" to capture the many aesthetic cases of awe in which power does not seem to be at work.

Accommodation refers to the Piagetian process of adjusting mental structures that cannot assimilate a new experience (Piaget & Inhelder, 1966/1969). The concept of accommodation brings together many insights about awe, such as that it involves confusion (St. Paul) and obscurity (Burke), and that it is heightened in times of crisis, when extant traditions and knowledge structures do not suffice (Weber). We propose that prototypical awe involves a challenge to or negation of mental structures when they fail to make sense of an experience of something vast. Such experiences can be disorienting or even frightening, as in the cases of Arjuna and St. Paul, since they make the self feel small, powerless, and confused. They also often involve feelings of enlightenment, and even rebirth, when mental structures expand to accomodate truths never before known. We stress that awe involves a need for accomodation, which may or may not be satisfied. The success of one's attempts at accomodation may partially explain why awe can be both terrifying (when one fails to understand) and enlightening (when one succeeds).

The more I think of it, the more impressive the whole thing starts to feel like, in the "memeplex that seems very effectively optimized for spreading and gaining loyal supporters" sense.