Idiomatic in this context is a synonym of "typical". It's possible to write Java with shorter class names, but they typically tend to be long.

"Up to expected standards" or "how it is generally seen" I suppose.

So for a more neutral example, CamelCaseClasses are "idiomatic" in Java, whereas classes_with_underscores are not. You could do either but the first is considered a more "correct" way to write Java, and is what you tend to see in the wild.