I take it to mean that with npm there is a very low barrier for entry. I mean it's not difficult to actually host an apt repo on your own server or on S3, but there's still more you need to know to get to that stage. I personally think that using something like apt for Node.js dependencies is a very good idea. I work with Node at work and on the side, and my/our usual stack is either Debian or Ubuntu. Even when Docker gets involved it's usually a Debian or Debian based image. [Edit: phrasing]

I think it only limits "freedom" in that it only makes sense on Debian-based operating systems. Whereas I can and do run npm on tons of different Linux distributions, MacOS, and (theoretically) Windows.

You can do things with deb packages on other Linux distributions kinda-sorta using various programs and scripts, but it's not painless. And Mac and Windows are right out.

The same applies generally as to why there are usually two types of dependency managers. One for binary packages and binary shared libraries (even though these often include source code for C & C++ stuff), and one for specific language development environments. See also Rust, Dart, Python, Ruby, Go, Lua, etc., etc. All those languages have their own package managers.

By default it does not allow to distribute and install multiple versions of the same library, and for good reasons.