It's not satire because you're not grasping simple gameplay similarities. Especially if your jump was to Malphite because I can already tell you a massive difference in how Malphite and Kayn will function: Malphite can only go in, Kayn has mobility to get him back out.
Now how would you say Kayn/Rhaast functions?
Dive in, hit the enemy some, dive out afterward. Repeat.
How would you say Rakan functions?
Dive in, hit the enemy some, dive out afterward. Repeat.
They're great team-buds because they're matching playstyles - they're dive team extraordinaires. Their key playstyle difference is in their roles: a support and a jungler.
But since you said "lategame diver is a class known for having a moderate amount of CC", then let's compare him to his most common comparison: Riven.
Riven's Q is a .5 second knock-up (if that) with a .75 stun on her W. Considering Riven's stun can be reduced, this is in the ballpark of about 1 second of hard CC built into her kit total. Riven has major mobility but is limited in her sustain, only having a shield in her kit. She has more burst on squishies but is basically screwed if stunlocked.
Rhaast only has CC on his W, a 1 second knock-up with a short wind-up (about .5-.75 seconds). Rhaast's cooldowns are rather similar to Riven's in the collective kit overall and both build down a similar path. Rhaast instead has significant sustain built into his passive and heals a bit off of his E, though that doesn't provide much in-combat utility. Rhaast is significantly effective against tanks and other bruisers but lacks burst on squishies.
So where does the CC come into play here?
Well, Riven's CC requires her to be within melee range, having a radius of 200 units tops giving it a grand area of about 400 units.
Rhaast's W range is 700 units. Its radius is unmeasured unfortunately, but I'd give it about a 150 radius. So we're talking about a significantly larger area of effect here from his supposed similar gameplay partner.
But he should be justified in having that then, right? Since he's all about the sustained damage versus burst.
Except that why wouldn't that just be .75 seconds? And why is it so fucking big of an area when his targets tend to be the hulking giants guarding teams? It's not like they're hard to hit.
And with that considered, why is it significantly larger than Rakan's AoE? His AoE knockup has a radius of 250, putting it at an area of about 500 units total around him. With the same delay, this is easier to dodge for approximately the same payoff.
Do you know why I point out Rhaast's knock up on his W out of his whole kit? Do you know why I'm not just saying "Why is Rhaast stronger than Rakan"? It's because Rhaast's knock-up is support-like. It's tank-like. In fact, I can point out significantly more tanks and supports with similar abilities than I can name assassins or bruisers with that kind of utility. It's because Rhaast's W isn't a bruiser or assassin's utility. It's quite literally - by comparison - a support's level of utility built into something that isn't a fucking support.