I don't necessarily agree with this.
Scala gives you the ability to write in an OO style - which can result it some funky looking hybrid code. But if you are disciplined, you can write perfectly pure, functional Scala.
Haskell is lazy and always pure, so writing hybrid code takes great effort. This improves code quality by making the path of least effort the best one.
Whether writing pure functional code is the goal is an interesting debate, but Haskell is certainly the better language to learn that in.
That's a key point right there. If you are an OOP guy trying to learn FP, Scala makes it too easy to stick to what you already know. I find that Scala's OO features get in the way of learning functional programming, because like you said, it requires discipline to do things in a functional way. With Haskell, you don't have a choice.