Reusing prtions of the gameplay code in a remaster can be feasible. Other remastering projects have managed to go further, although they had more favorable starting conditions. The remastered Full Throttle game is running the original SCUMM game logic without any changes. They instead extended the VM to perform on the fly asset swaps for the remastered artwork and sound.

> Did player expected them to package the optimized lisp virtual machine they had back then

Well, yeah? It's a remaster, not a remake. A remaster implies taking an existing project and updating it. And it makes sense to keep the old game logic - why write from scratch if you already have the whole game in a working state?

It's a remake, not a remaster in the definition that you are thinking of. Vicarious Visions did not use any of the code, likely to the fact that either it was unusable or they did not ever have the source code.

N. Sane was done in Alchemy Engine, not Unreal Engine 4 (unlike Reignited), although VV is likely following suit with their upcoming project.