It's a new game.
By remaking it instead of just copying the old one will give the engineers more opportunity to expand on the code later. Expanding on KSP1's code to enhance the vanilla experience is not necessarily that easy, and it undermines the purpose of making a sequel.
It's simpler to create your own content rather than relying on someone else's groundwork.
that's not him proofs though, what you say is wishful thinking (a fallacy) unless you have some proof.
I do hope that the physics engine is brand new as KSP's was showing it's limitations but seeing the wobbly rockets and the patched conics it seems like an improvement, not a complete rework. Similar to the terrain system apparently, until we get the new one.
Morrowind, Oblivion and Skyrim share a lot of code even though they are different game, just by the fact that they share the same engine.
This is downright wrong. Just by knowing the mistakes made before and how they were made eases drastically the work.