What?? I'm thinking. Ruby can be quite expressive, even beautiful.But Tom can't be sure what Ruby is going to do. Some imported code could be modifying methods on built-in classes. You can never be sure exactly what will happen when this Ruby code executes.
He's right about that. "Readable" isn't the word I'd use though: Ruby isn't "reason-aboutable." You can't be completely sure what it's going to do without running it. (No wonder Rubyists are such good testers.)
Tom agreed that Ruby could be good at expressing the intent of the programmer. This is a different goal from knowing exactly how it will execute.
Stricter languages are easier to reason about. In Java I can read the specification and make inferences about what will happen when I use the built-in libraries. In Java, I hate the idea of bytecode modification because it interferes with that reasoning.
With imperative code in Java or Python, where what you see it what you get, you can try to reason about these by playing compiler. Step through what the computer is supposed to do at each instruction. This is easier when data is immutable, because then you can trace back to the one place it could possibly be set.
Beyond immutability, the best languages and libraries offer more shortcuts to reasoning. Shortcuts let you be sure about some things without playing compiler through every possible scenario. Strong typing helps with this: I can be sure about the structure of what the function returns, because the compiler enforces it for me.
Shortcuts are like, I can tell 810 is divisible by 3 because its digits add to a number divisible by 3. I don't have to do the division. This is not cheating, because this is not coincidence; someone has proven this property mathematically.
Haskell is the most reason-aboutable language, because you can be sure that the environment won't affect execution of the code, and vice-versa, outside of the IO monad. Mathematical types like monoids and monads help too, because they come with properties that have been proven mathematically. Better than promises in the documentation. More scalable than playing compiler all day.
"Readability" means a lot of things to different people. For Tom, it's predictability: can he be sure what this code will do? For many, it's familiarity: can they tell at a blink what this code will do? For me, it's mostly intent: can I tell what we want this code to do?
Haskellytes find Haskell the most expressive language, because it speaks to them. Most people find it cryptic, with its terse symbols. Ruby is well-regarded for expressiveness, especially in rich DSLs like RSpec.
Is expressiveness (of the intent of the programmer) in conflict with reasoning (about program execution)?
 "How do you really feel, Jess?"
We want to keep our programs simple, and avoid unnecessary complexity. The definition of a complex system is: the fastest way to find out what will happen is to run it. This means Ruby is inviting complexity, compared to Haskell. Functional programmers aim for reason-aboutable code, using all the shortcuts (proven properties) to scale up our thinking, to fit more in our head. Ruby programmers trust inferences made from example tests. This is easier on the brain, both to write and read, for most people. It is not objectively simpler.