As I explained in another comment, Scheme is indeed dynamically-typed (like Nix), but it's also strongly typed: one can define new data types, they are disjoint, and there can be no casts and the like. In Guix "packages" and "operating systems" are different types, you get a type error if you use one instead of the other; in Nix both are "attribute sets" (key/value dictionaries.)

I think the one-type-fits-all ("attribute sets") vs. strong typing (disjoing record types) axis matters more than the static vs. dynamic axis here because most of the code (OS config files, package definitions, etc.) is loaded dynamically.