Your post appears to characterize any such IDL as a "mistake". What makes attempts to formalize an API in this manner inherently problematic? Or did you just mean that it's a mistake to do a different IDL for REST-specific APIs instead of using an extant one? Does your complaint extend to non-web IDLs like Thrift or protobufs?

In my experience, there is nothing inherently wrong with this type of IDL. Like all architectural decisions, it comes with its own tradeoffs, but there's no reason the tradeoff profile is inherently wrong.