This is a bit of a devil's advocate question... but can anyone educate me as to the advantages of testing DLC in a completely hidden environment? Why would they not want us to know they were working on something? Wouldn't it just increase hype and interest in the game - or are they worried about hurting Platinum Edition sales?
Avoiding leaks must seem like an obvious one but as I recall GS was leaked quite extensively before the announcement, which had nothing to do with SteamDB...
You can ask this question about any kind of obscurity and why it has such degree. Why not outright tell from the start what they are working on and updating us on-the-go of what vision worked, which didn't, what's new in creative process etc. Wouldn't that give even more hype than just saying they do work no something?
LoL, for example, while not too transparent, is much more open. Prior to release of Senna we already knew we'll be getting Non-traditional Marksman. With her release (Supportive Marskman), we learnt next releases will be "Lunari that changes weapons like the moon changes phases" -> "Ionian underground fighter that absorbs damage as rage and then bursts it all back, destroying his enemies with bare hands" -> "Fiddlesticks Rework" (they outright showed us what he'll look like and teased he'll have ways of scaring the poo out of enemies) -> "Volibear Rework" (showing his concept art and proclaiming focus of God of Thunder theme) -> "whimsical jungler" -> "edgy midlaner".
Senna has been released for few weeks, this week we'll be getting Aphelios, the Lunari with many weapons. That gives you idea that the edgy midlaner will be released as new content somewhere in Summer of next year. And yet he's already announced. What do we know about him? "edgy midlaner". That's literally it. Because they theirselves aren't so deep into the process to be sure what theme they'll land on. We got display of Fiddle's model despite the fact he should release at around end of Jan start of Feb of next year. No model for Voli because it's not finished yet, we only know his concept art. There alread were official teasers of the fighter's abilitiey names or actions.
So all it boils down to is developer's choice of how they build hype. Civ wants it fully covered and then burst all the cards, Riot periodically gives "Dev Updates" where they outright explain what choices they considered for theme, what they landed on, how they want it to translate to gameplay, and even that is only one per month or two months.
All it further boils down to is confidence. You may retire some announced idea because it won't work and people who won't understand it had to be scrapped will be de-hyped and go "Man, I was so looking forward to that". Or you don't think it looks good now prior to finish and feel like people will always judge based on this half-baked product rather than the finished one. Or If your company is all over the place project might be canceled all together and trust damaged. I suppose given that 2K is the master of coin for them, they might not be fully confident in their project's exact release form and would rather wait until sure.