So they couldn't say take previous code and build on top of it say a new game and leave elements of that said previous code for I don't know EA? Well call this code placeholders where they will replace that eventually down the road.
Examples for placeholders would be the way they handle inventory, some of UI, the basic player profile approach (having save files connected to profiles), the re-used containers and artworks - things that can be plugged in and work even if the mechanics of the game are different. So for example you don't need to worry about how the weapon stats work as long as they have a weight and an icon you can put them into the inventory (and containers) probably with little change (if any at all) to what they had in DOS2-code.
When it comes to gameplay, DOS and D&D work too differently in detail (in the sense that you ain't damaging proctetive values before you can hit health, the abilities are not on cool-downs but connected to D&D resources in-between-rests,...). So when you want to have barrels & surfaces the changes might not be huge to the code, but in truth adding these things is simple in the first place. Wether you copy and paste them or just rebuild them to fit the new code structure won't be a big difference (I would assume) - the bigger question is how or if you would use them and this is where DOS2-mentality comes into play. You had explosive grenades in BG2 as well as far as I remember, but they were not omi present and did not offer as waste possibilties to cheese the game.
If you are talking about placeholders in the sense of 'balancing left to do' then I see what you mean. But in this case I would go and call most of the EA as placeholder since it might be tweaked