What makes you think we'll get atmospheric planets in 2020? The leak explicitly mentioned no indication of atmospherics. Legs and base building is an enormous update on its own, let's not get carried away.
While this is true, as has been established, the leak is definitely not an exhaustive list. For example, there was that rumour that came up elsewhere where somebody allegedly spoke to David Braben, who confirmed the codebase would be getting refreshed in the 2020 update... that's not there in the leak. If that's true then that's also clearly a massive undertaking. If everything's true, then we're looking at a refreshed codebase + spacelegs + base building... why not throw atmospheric landings in there for good measure too?
Let's not forget there was also that other thread that showed the time period for the New Era from going into "full production" to its eventual release sometime at the end of next year (assuming no delays), is pretty much on par with production of a full game. So I don't see it as inconceivable that Frontier could be refreshing the codebase in order to more smoothly incorporate legs and atmos.
The one thing that concerns me with a refreshed codebase is... are we going to lose our progress? How much of a refresh is it? Can they do it while leaving our save games alone? My absolute worst nightmare would be the announcement of "good news everyone! we're refreshing the codebase to make the experience the best it possibly can be; bad news is we've got to reset the galaxy entirely".