That's kind of like saying you can write immutable data structures and then bolt on STM and try to force yourself to stay in functional territory as much as pos

That's kind of like saying you can write immutable data structures and then bolt on STM and try to force yourself to stay in functional territory as much as possible in any language, which the paper addresses. You can do this, and Rich did it in C#, but in my experience it's not long before you start to have clashes of philosophy with other libraries you depend on and with the language itself, not to mention people you work with. Believe me, I tried on one project to write as Clojure-y in Ruby as I could, and as soon as you reach for a library you need to interact with that doesn't play by the rules, things get weird if you want to keep things functional and immutable all the way down. You need to flagrantly violate established idioms in the core of the language, and often you wind up reimplementing things in the core of the language. People who aren't familiar with why you wrote it a particular way will come in at some point later and imperative or OOP it up.