Zero to Nix is opinionated because it advocates learning and using flakes and treating channels as deprecated. Zero to Nix also deviates from official recommendations in that it recommends installing Nix using Nix Installer rather than the official Nix installation script.
https://zero-to-nix.com/about#whats-opinionated-about-zero-t...
> Opinionated software means that there is basically one right way to do things […] Non-opinionated software leaves lots of flexibility to the user https://stackoverflow.com/q/802050
Providing only a single answer for how to do things is helpful for beginners who haven’t yet formed a basis upon which to make informed decisions.
'Opinionated' here means that the authors are trying to spare newbies from the 'paralysis by analysis' problem by making some (defensible) choices about what conventions to use on the reader's behalf up front.
These docs are going to 'lead' the official docs in the sense of coming out first because making things official will involve standardizing some of the interfaces used here and solidifying more consensus around them. (There is lots of organizational and technical work happening around that right now. You can follow along by looking for the minutes of various community team meetings on NixOS Discourse and following links from there, if you're curious.)