They are not deadlines, they are targets. There is a big difference.
Deadlines only exist when there is a publisher and a contract that says "We agree to deliver [X] by [date] or else we are in breach of our contract." They are a necessary part of that kind of business arrangement, but no such arrangement exists here.
CIG aim to have things out by whatever date they picked. That is what they build their schedule around. That is how they plan ahead. But it is not set in stone, and the reality is that every developer misses their targets constantly, because that is how game development works. Any time you create something new, you run into unknowns. The difference is that other developers resort to handing over something less than what they had planned, in order to remain within their time constraint. CIG are not willing to do that. They have the budget to play the long game. They are committed to giving the people the game they promised. Without compromise. Because that is exactly what a lot of backers paid them to do. Because they are sick of seeing bad games made to meet profitable release dates. Shipping an inferior product in order to meet an artificial deadline would be a complete betrayal of the core promise of Star Citizen.