I'd be surprised too if the new planetary improvements are locked behind the Odyssey DLC paywall.
After all, if you don't want Horizon players to walk out on foot all you have to do is deny access to the ship's walkway ramp and prevent people from exiting their SRV.
Very much doubt you can access the planetary base interiors without having to walk in. Unfortunately, this will probably apply to orbital stations and outposts too.
Are you sure there will be an option to leave a ship on airless planets in Odyssey? Those suits look pretty thin. Also, I didn't see any proof that space legs will be avaible for Horizon's planets.