For the presentation, they should open the gates of an empty hangar.

Their landing page contains literally a picture of an empty room.

https://www.northropgrumman.com/what-we-do/air/b-21-raider/

But that would spoil the dad joke where everybody pretends they can't see it.

The US military's strategy seems to be to give potential adversaries just enough info to make them think twice about attacking, but not enough to mitigate the threat. Russia, China, and the US know the others have nukes, but they don't know where all the nuke platforms (mostly subs) are or what their capabilities are, so there's an unknown upper bound of potential retaliation.

And so it is with stealth. We know there are subs armed with powerful nukes, but we don't know where they are, or what cities they've been instructed to flatten in the event of an order to strike. Stealth planes are just nuclear missiles that fly up to the target.

As long as it's not revealed by radar...

...and even if it is revealed by radar, if it's revealed at a smaller radius or if the weapons radar has difficulty locking on or if the highly sensitive radar that is capable of seeing it is more vulnerable to jamming, those are all benefits.