The title makes it seem like they're putting these pics on the Steam page. Or at least that's what it seemed like to me.
But they're actually sneaking these pictures into these capsules you can find in the game. Normally these capsules have screenshots from other players' games. Now players have found screenshots that are from the sequel (presumably). That's really cool!
Unknown Worlds Entertainment is currently making another Subnautica game. They are publicly referring to it as "Subnautica 2." So apparently they consider Below Zero to be a standalone expansion to Subnautica a bit like Half-Life: Opposing Force rather than a full sequel.
A few things that I have heard the developers confirm about this new game:
It will take place in the same universe as the first two games.
It will not take place on planet 4546B
There will be swimming in it
There will be submarines in it. Plural.
It will be co-op capable. If I understand what they've communicated right, it is going to be a single-player game that will have a "join game" button so you can invite a small number of buddies to join you.
From the screenshots they've shown, there's going to be colorful ocean wildlife in it.
According to the Wiki, one of the original concept artists and the composer for Below Zero's soundtrack are working on it.
They're playing a lot of details close to the chest for now, very few gameplay or story concepts have been discussed. It is very likely going to be an ocean survival game with a nonzero chance of having your submarine bitten off by a 300 foot scream eel.
BZ was never Subnautica 2. It was a spinoff produced by a different team. It was originally supposed to be an expansion for the first game, but it got big enough they decided to release it standalone.
I liked Subnautica 1 but I feel like it took me way too long to find a few key blueprints and I'm not sure if it's because the game is designed weird or if I just couldn't find them.