It's been a fundamental element in the plan from Day One. David Braben himself has stated that this is his single favorite planned feature in a dev video for the future of Elite Dangerous.
Back in 2012 when FD needed the money they would say anything ... Things change over time, more so now MS$ is on board
Now, from an addon perspective I hope FD take their time with it and make something better than they originally produced for Frontier - back then it was great (in terms of achievement) but ultimately it was boring as hell. With the increase in computing power FD have the potential to make it awesome, but IMO EVA (walking about) would be a better 1st add-on pack as then when Planetary Landing is completed you get also get out your ship.