It's a bug then because in actual practice, even with the content type of episode it acts as generic, with only the title and description being populated like the componentguide says it does.
I just know because I had to set the content to episode, and the descriptionstyle to movie to get it to display the runtime of a video file. Generic and 'unspecified' descriptionstyle prevent the runtime from displaying too.
Here's a relevant thread I read a while ago.
"The "Length" content meta-data parameter will display on both "video" and "movie" description style poster screens. Please check that the value of your "Length" parameter is an integer in seconds. The screen takes care of displaying in hour/min format, but you need to supply the Length parameter as an Integer in seconds.
Release Date and rating only display on the "movie" description style, they do not display on the "video" description style.
Which would indicate to me that you're not supposed to be able to get the Release Date to display unless you explicitly specify the "movie" description style somewhere for the springboard screen.
Or perhaps I misinterpreted what he wrote?