Not exactly. If one package needs foo as a dependency, and another package also needs foo, it won't download a second copy.
But if another package needs, say, an older version of foo, it can download the older version of foo and you'll have two foo packages.
This is possible because Nix packages have hashes to differentiate between them, so any package can ask for the exact version of foo it needs instead of asking for foo in general and hoping the version it gets isn't incompatible.
Pretty sure this is also different from other containerized package managers, like Flatpaks or Snaps, which I believe throw all the dependencies in with their packages so each package has exactly what it needs in its container, and which is obviously going to be much more bloated than having shared dependencies. As far as I know Nix on the other hand doesn't get any more bloated than other distros (keep in mind that regular distros like Arch will still sometimes have multiple versions of a package, for example Python).
The problem is that nothing Red Hat has done justifies them breaking the rules.
Have they made tons of contributions back to open source? Yes. Do they need to make money? Yes. Are there organizations and people who are, in essence, freeloading off their work? Yes.
But here's the thing. At the end of the day, they chose to make their project open source and to build it on Linux. And that choice comes with rules that they (and everyone else that have used Linux or other FOSS projects) have to follow, no exceptions. You can argue that their motivations for wanting to do so are understandable all day long. You can argue the GPL is bad and shouldn't work this way. But they still chose this ecosystem.
Now, have they actually violated the GPL? We'll leave that up to the lawyers to decide I guess. But if we're only talking whether they should be allowed to violate the GPL, the answer is absolutely not. If they didn't want RHEL to be open source and stolen by freeloaders they should've made their own operating system with their own license.