They really haven't, though; it's not supposed to be playable. It's "pre-reformed", which means that it has all the penalties of being unreformed but doesn't have the benefit of being able TO reform. It's simply a placeholder religion for historical characters, much like Roman culture is.
As has been suggested, if you really want to play a resurgent Hellenism, you should either download a mod or make your own. Otherwise the religion will never be fleshed out enough.
It's not fully pre-reformed though in a sense.
You can ask characters to convert, and it has a 20% organized religion moral authority bonus.
On the other hand.. It cannot make holy wars.
I don't like to use mods for something that should be available by default. I never asked for the religion to get it's own mechanics.
But it should be functional.
Similar to how Norse was before Old Gods was released.