All good points but it’s worth noting that in order to install any non-free NixOS packages there is an explicit flag you need to set which affect their entire OS or set of packages on your machine.
That said, downloading a jar instead of compiling the jar from source is definitely a notable difference and potentially is unfree software despite the license stating otherwise and the flag previously mentioned set.
While there is nothing inherent about NixOS To prevent non-free software from entering the ecosystem, there is also nothing inherent to Guix either that would prevent any non-free software. Ultimately it comes down to community curation of the packages and while Guix almost definitely is more vigilant about free software both have focus on free-software
NixOS has the option to install non-free (which you need to explicitly enable to install non-software) and Guix is more strict where their official packages don’t intentionally support any non-free software and they employ practices that are more likely to catch oversights.