Lets take just one aspect of 'ship interiors' that he previously alluded to:
overlapping physics grids. In short, the idea is that you are in a spaceship (with its own physics grid), that itself is moving in a larger physics grid. How to deal with this?
1) Only allow walking in ships when the ship cant move, AKA a 'static level'. At this point the ship doesnt need its own physics grid. This would be the case when you can only walk in crashed ships, or docked/landed ships. Think Mass Effect. This is the easiest, and at this point a 'ship interior' is not much different from a random building or set of rooms.
2) Always allow walking in ships, and 'hand over' the player between different grids when needed. This is not trivial to do. Star Citizen has been trying to add this to CryEngine for literally almost a decade and it is still extremely buggy. In this case the player frequently just dies, or gets catapulted 100000 km or whatever when you leave a grid or when grids need to interact. This is the 'doing it realistically' options that studios typically try to avoid at all costs because it is a huge amount of effort for relatively little gain.
3) Always allow walking in ships, but complete separate the interior/exterior. This is how Warframe 'fakes' it. You are in your own little level, and the 'outside' is at best projected on the windows but not 'really there'. This might seem to be the best of both words, but it does bring limitations (you can really shoot from outside a ship at someone inside, for example) and makes concepts like 'boarding' tricky as you need to hide a loading screen somewhere. This is the 'intelligently fake everything' design school, which is generally considered a very good approach if you want to get a lot of 'bang for your devbucks'.
So already just one choice could add many, many years to the development, shorten it drastically or create further interesting design choices. If you want to seamlessly enter the interior of an Anaconda wrecked and floating in space, and the ship is spinning/pushable, that is not easy to do. If you drop the need for 'seamless loading' and its okay to just load at some entry point it becomes vastly easier. So it is impossible to estimate anything at all. But I can say that if someone were to argue FD would definitely need to go for #2 and definitely only add 'cosmetic stuff', that person is simply painting a worst-case design scenario for obvious ulterior motives. Which is really not needed.