My best answer is: if they get to sufficient scale, both Lemmy and Kbin will face scaling issues to get through, but Lemmy is based on something that will make it much easier for humans to get through a lot of those bottlenecks.
I hope what this answer conveys is that the technology choice is a major factor, but not the only factor. If the Lemmy dev team doesn’t know how to scale a service, and don’t enlist the help of people who do, the underlying technology won’t make much of a difference. But it does give them a very strong upside.
Another Lemmy user was saying that the Kbin move to use PHP was like someone saying: “oh, I like the airplane you just built by yourself with the intention to fly above the clouds, I’m going to do the same thing, let me prepare my cardboard”, and there’s a lot of truth to it. 😉