they didn't create a fork under their control

I'm sorry but this is simply incorrect (See 1,2,3), as I have previously stated. You could point to sources that agree with you though if you disagree.

1: https://itsfoss.com/librewolf/

2: https://wiki.gentoo.org/wiki/LibreWolf

3: https://lwn.net/Articles/1012453/

These are some examples that use "fork" in describing Librewolf.

What they are doing is customising the current code of Firefox at the time of compiling the LibreWolf project.

You have described the creation of a fork.

... I'm not going to continue a fruitless argument.

I'm here if you wish to discuss further.