Dependency injection subjugates readability and simplicity of data flow for a narrowly defined notion of testability. Even in languages like Java, it's possible to substitute class definitions for mocked ones without having to thread a weird sort of test monad through code that otherwise serves a purpose.

I pretty fundamentally disagree with making production code more complex to make unit tests easier to write. Make the tests more complex instead and think about the natural units for testing. Maybe the natural unit for testing isn't exactly one class.

This is counter to the conclusion of the article: it's true one shouldn't force Java idioms on Ruby but also if the Java idiom doesn't translate well, maybe it's a bad programming idiom in general.