Forms in UI code also tend to have a bunch of business-logic type validation (e.g. "there's already a user with that email address") which schemas alone are unsuitable for.
Well, they produce JSON though. I understand that there is much more context related to the user interface but you can extend JSON Schema to be able to tie more information to it. It's an incredibly flexible & collaborative way to define the schemas. The backend people are also familiar with it and can validate the data & collaborate with frontend people over a JSON Schema probably much easier compared to a frontend libraries DSL.
They definitely are using their DSL for things far out of the scope of JSON schema. The builtin debouncing is pretty neat: https://formkit.com/essentials/validation#debounce-milli
Forms in UI code also tend to have a bunch of business-logic type validation (e.g. "there's already a user with that email address") which schemas alone are unsuitable for.