Maybe 'rolled their own' or 'first invented the universe' would have been slightly better.
But I do feel like they probably know what they're doing enough to have a more modest version of success with this other project, i.e., meeting their own needs well without burning up too much money or time. They're still a really small, focused company, and they have a lot of relevant experience.
And for every Rails out there, there are thousands of internal frameworks with big ambitions that just turned out to be inferior to what's already available. You just can't know it when you start developing. It takes a bit of ego and ambition to go that path, but sometimes it pays off. And my guess is that if it paid off in the past, you're more likely to try it again.
AWS ECS -> Google GKE -> AWS EKS -> Self hosted Kubernetes -> bespoke solution.
They are pretty much changing their tech stack every few years, never maturing their knowledge of the current tech stack.
I am sure it makes sense to self host, but given their track record I am wondering if they wil be migrating to the cloud again in a few years.