Serious question:
What are the odds they show off a spin-off game?
I just don't think 343 has been working on Halo 6 only since the release of Halo 5. We know that they are a big studio, and while a lot of their staff isn't all in game development, they have to have quite a lot of devs there.
Now previously I would have assumed that there was a small team focused on Halo 6 right around the time Halo 5 released that got ramped up as the DLC for 5 winded down mid 2016, but I'm kinda split on that. Mainly because while the DLC for 5 was great, the monthly release schedule and actual content of the releases make me think that there were not many devs dedicated to Halo 5 DLC. Just a few small dedicated teams like the forge team.
Then there's Halo Wars 2, which while bearing the Halo name, I am guessing isn't taking up a lot of devs on 343's side given Creative Affinity's involvement in the project. Its possible CA is doing most of the the game dev side and the 343 staff on the project are there for story and art design related reasons.
This all being taken into account (along with a grain of salt) paints a messy picture, but I just get the feeling 343 has a team that's working on something entirely separate from Halo 6.