So here's my two cents, unsolicited though it may be.
Could this have been a mistake? Sure, it's possible.
But speaking as a Lawyer, a Publicist, an entertainment industry professional, and a game designer, the number of screw-ups that have to happen for that to be the case make it less likely than a deliberate leak.
1) the camera was shooting b-roll unrelated to what was being discussed in the audio. So it wouldn't have been given free reign to wander. It was only shooting exactly where it was supposed to, and exactly what it was supposed to shoot.
But what if it was given free reign to go wherever? Well then, anything that wasn't explicitly allowed to get out would have been taken down or covered with a drape. This isn't a small, indie studio, this is Firaxis who publishes through 2k.
But let's say that it slipped through and made it onto the tape.
2) if anyone, literally anyone, from the design team reviewed this video before it was approved they would have noticed the pictures, and called shenanigans right then and there.
But maybe they figures it was safe to let it slide cause they were blurry and stuff?
Nope. That team knows this community, and they have watched it dissect with a scalpel every second of footage that has been released. I'd be shocked if anyone at Firaxis with enough knowledge to be the yay-or-nayer on a video release could possibly make that mistake.
But if no one reviewed it at all that could explain it right?
3) if no one at Firaxis reviewed the video at all, that could explain how a mistake if this order could happen. But then *someone* is probably on the chopping block for releasing un-reviewed video content.
And let's not overlook how perfect this "slip" was.
4) 1 second of video footage that "looks harmless" but provides just enough information, with just enough give-aways and organization to the layout of said info to allow a dedicated team of superfans with access to the internet to play a fun and satisfying ARG for a day without officially spoiling any unknown content.
The other possibility is that while it wasn't explicitly planned, it was let through on purpose once it was caught. That splits the difference between oops and planned I suppose, but in the end it is more the later than the former.