The dev claims they are making a clean room replacement not a decompilation.

Clean room usually means having one person/team reverse engineer a product, write a detailed specification and have another person/team implement that specification so that it's not contaminated with copyrighted material.

I have my doubts about the clean room part but it's believable that it's not a decompilation project that aims to come close to the original code structure.

Desloppiled Code is just going to be a huge mess every time I'd imagine.

For sure. No human is ever touching that code.