It does enable code that "hides" implementation very well, like the quoted examples authentication API lets you authenticate in any way you can imagine, as in literally any way imaginable.
Its incredibly flexible. Want to only be able to send the request out after you've touched a file, send of a Message through a message broker and then maybe flex by waiting for the response of that async communication and use that as a custom attribute in the payload, additionally to a dynamically negotiated header to be set according to the response of a DNS query? yeah, we can do that! and the caller doesnt have to know any of that... at least as long as it works as intended
Same with the Proxy layer, the client is _entirely_ extensible, it is what Inversion of Control enables.
It just comes with the unfortunate side-effect of forcing the dev to be extremely fluent in enterprisey patterns. I dont mind it anymore, myself. the other day ive even implemented a custom "dependency injection" inspired system for data in a very dynamic application at my dayjob. I did that so the caller wont even need to know what data he needs! it just get automatically resolved through the abstraction. But i strongly suspect if a jr develeoper which hasnt gotten used to the java ecosystem will come across it, he'll be completely out of his depth how the grander system works - even though a dev thats used to it will likely understand the system within a couple of moments.
Like everything in software, everything has advantages and disadvantages. And Java has just historically always tried to "hide complexity", which in practice however paradoxically multiplies complexity _if youre not already used to the pattern used_.
