Isn't it fairly easy to slim down the runtime at build time to only the methods you're using? I was under the impression that this was possible, though I've never actually done it so what do I know....

Yes, it is. This is commonly accomplished with a tool called ProGuard. Doing this comes with some other trade offs but it is easy to do.

Just because they didn't do their research doesn't mean it's not a no-brainer.

> it requires build complexity

All languages benefit from not having to deploy their standard library on every iteration, not only Scala.