Well... Saying "nothing done from scratch" is hugely unfair.
The developers wanted high compatibility with original Dune II, so they used disassembled and decompiled sources of original Dune II as a base. It was a lot of work to decompile.
Then, Dune II is a DOS game (INT 0x10 anyone? ), so they needed to write DOS to SDL translation layer, which is being gradually phased out as parts of the decompiled code are being rewritten to native.

It is a lot of work which gives them a lot of kudos.