Eh, I think it's that with Nixlang one can write

  foo.bar = true;

  foo = { bar = true; };

I've never seen more than one level of nesting in a piece of Guix code, but that comes with the caveat that Guix is quite happy to extract fields that would perhaps be nested with other languages into their own top-level fields, like the build-system and arguments fields being separate in package definitions.