I read that message to mean clojure/core.[library] and not clojure.core. It's a criticism of the slower development pace and low visibility that libraries like match, logic, combinatorics, test.check, data.zip, rrb-vector, cache, etc. have.
The author can clarify but if that's the critique its not without at least some merit.
Yes, this is part of it (although I do include things like core.async, reducers/transducers etc in that). A lot of Clojure feels like a bunch of thesis projects tacked together and abandoned. Most of it was quite innovative and cool at the time, but it's not being iterated or elaborated on. That's nobody's problem, really, I just happen to find it frustrating.
I'd almost be encouraged by the rework going into spec if it wasn't happening at such a glacial pace.
The author can clarify but if that's the critique its not without at least some merit.