The reason is that builtins.fetchTarball, just like builtins.fetchGit and builtins.fetchurl, is not a fetcher but a built-in function (primop). (Yes, it is confusing that Nix imports many builtins into scope automatically.)
Fetchers are actually just functions that produce fixed-output derivations. They utilize various builders (build scripts) to achieve that. For example, fetchurl fetcher is defined in pkgs/build-support/fetchurl/default.nix and its builder is next to it.
The builtins (e.g. builtins.fetchurl) are defined in Nix source code. One difference is that the built-in functions cause download during evaluation, which is bad for all sorts of reasons. Compare
nix-repl> builtins.fetchTarball { url = https://github.com/NixOS/nixpkgs/archive/nixos-20.09.tar.gz; sha256 = ""; }
[…starts downloading immediately, blocking your script, eventually returning a store path]
with
nix-repl> fetchzip { url = https://github.com/NixOS/nixpkgs/archive/nixos-20.09.tar.gz; sha256 = ""; }
«derivation /nix/store/x4r0sqgzkjm36lil5i4dkifgs5ipwkzr-source.drv»
The latter returns a derivation almost immediately, delaying the fetching (and extraction) to only when you build the derivation.
I agree that a brief mention of the fetching builtins (maybe just a link to the Nix manual) would make sense in the Nixpkgs manual.
Just for completeness, builtins.fetchTarball serves the same purpose as fetchzip – download an archive and then unpack it into store. Despite the name and docs, both support tarballs and zip files. (Nix uses libarchive, and fetchzip depends on unzip and has gnutar available from stdenv.)
To make the fetchurl even more confusing, there are few more objects with the same name:
fetchurl depends on stdenv, which in turn uses fetchurl to obtain sources of its various dependencies. For that reason, there is also a simpler fetchurlBoot function that is used to bootstrap stdenv.
fetchurlBoot function invokes fetchurl.nix file from Nix, which creates a fixed output derivation directly without a stdenv. To be able to do that, it uses a special value for the builder attribute – builtin:fetchurl (no relation to the primops). When nix sees it during build, it uses custom code instead of executing the builder script as it would normally.