there is no mention of that anywhere on their website.

A custom version of Firefox, focused on privacy, security and freedom.

This project is a custom and independent version of Firefox, …

LibreWolf is a free and open-source fork of Firefox, …

This repository contains all the patches and theming that make up LibreWolf, as well as scripts and a Makefile to build LibreWolf. There also is the Settings repository, which contains the LibreWolf preferences.

They take Firefox, make changes to it, then release it. As such, it is a fork. More specifically a “soft fork” since they continue to pull changes from upstream (Firefox).

EDIT: Oh I see you’re focused on the “duplication of the code” part. A bad phrasing on my part. It doesn’t matter the specifics of how they pull in the source code, it is pulled in and used as the basis for librewolf’s modifications.

They could even pull it in on first launch and compile the latest version of Firefox with their modifications for subsequent launches and it would by all means be a fork, since they are shipping a modified version.

LibreWolf is not a fork, though.

It certainly is.

They duplicate the code, creating a “fork” under their control, and make independent changes to the code. That is all that is needed to satisfy the “fork” definition.