The section on forkIO vs forkOS is interesting because the author does not mention implementing green threads atop the JVM. It would be feasible for Frege and its core libraries to use an implementation of "green threads" based on Promises or CPS under the covers in order to implement a concurrency model indistinguishable from Haskell's.
While it is true that the vast majority of JVM code you interact with is blocking and expects to own whatever OS thread it is running on, that could be treated as a concern of FFI, where untrusted/blocking code in Java land gets dedicated OS threads (similar to FuturePools/execution-contexts for scala Futures, or how you would manually isolate blocking code for Erlang or Go FFI.)
Additionally, the comment dismissing STM seems like an afterthought, because there are at least three JVM implementations (Clojure's, Scala's, and Multiverse)
But the main comment I was referring to was:
To do serious concurrent work in an JVM environment is
different and it will thus require different knowledge
... which suggests that the author isn't interested in supporting Haskell's idiomatic concurrency model atop the JVM, and is instead fine with the status quo of blocking code.