Unless I’m misreading the code, it looks like it’s running GA, but giving it an opt-out signal to harvest.

It's not, It's adding a global variable called _gaUserPrefs to every site. If the actual GA script is loaded it would look for this as the opt-out signal.

Of course any other tracking (or GA tracking) could use this as a part of fingerprinting.

Ah. So there is a "trust us" aspect to this.

Yes, if you use googles products to block googles products you have to trust google.

My main point is that the extension itself does not load GA, which the parent seemed to say. It can also be used for other fingerprinting since it is a variable accessible by other scripts.