Whether you can get away with doing things unannounced depends on how much attention the company gets. Some companies have a lot of journalists watching them and writing about everything they do, so when they start doing A/B testing there will be stories written about the new feature regardless. Better to put out some kind of announcement so the journalists write something more accurate? (This approach seems pretty common for Google.)

Similarly, many game company can't really do beta testing without it leaking.

OpenAI is in the spotlight. Hype will happen whether they want it or not.