idiomatic? ( I am unfamiliar with the meaning of "idiomatic" in this context, I suspect this is programming language jargon? )

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.

Maybe... personally I find most of the carping about Java class names to be hyperbole. This is one of the more extreme examples you'll find (especially in common use) and, again, if you look at what it actually does, the name is descriptive and reasonable.

As to whether or not the whole persistence mechanism is "over-engineered", well... that's a separate question. I can see some of the arguments for it, but when I look at how much easier my life is with JPQL and Spring Data and the auto-generated JPA classes, versus hand-coding raw JDBC calls and raw SQL, I'll take the former every time. And I, for one, like transactions, and the way Spring lets me annotate my Service Facade classes to manage transactions for me.

Oh, the names are always descriptive and can be justified. You don't see things like "VeryNiceClassLoaderClassThatIMadeInJuly", it is only ever things that are justifiable.

The problem is that when you read modern code in other modern languages everything tends to be shorter, still descriptive, and still justifiable. It is something that you can't help but notice if you regularly use multiple languages.