Retopology means polygon reduction, it's only a trendy word for an old technique. Retopology is not about reconstructing polys from an existing clean model. That would be simply called a reproduction, or a derivative.
In the case of LFS mod, 'retopology' as it is used, is a form of plagiarism, it is no gray area, it is just fully wrong when it comes to licenses that forbid reproductions or derivatives.
"Plagiarism is the representation of another author's language, thoughts, ideas, or expressions as one's own original work." - Wikipedia.
By the way, do you realize that any imported object wich has wheels in its mesh, needs them to be removed for making a mod in LFS, and as such, the mod will be a derivative work simply because the original mesh has been changed ?
Yes, it's very unlikely that an author gets angry from having his object's wheel remove to make a mod, but still, it's a breach of "no derivatives" licenses.
So if u want to be really clean, the only choices are private agreement or public domain / CC BY / CC0 and some other uncommon free licenses like "license art libre" (https://artlibre.org/licence/lal/en/), wich explicitely allow reproductions and derivatives.
edit :
it is also wrong in LFS to forbid derivatives when original model is CC BY SA or equivalent.
As it is now, LFS is breaching the SA clause.
CC BY SA want us to name the author and share any derivative work with the same license, so derivatives must stay allowed in LFS, as the original license states.