I'm not really sure what the hope here is for the people in the thread. Okay so the author makes it opt-in by default. The first user downloads the tool, tries to run it, goes "oh, well that's useless" forks the repo deletes the header check and runs it. Now to block them you have to employ "big league" blocking techniques instead of setting a header.

Having it be opt-out actually makes it so server operators have the ability to turn it off because a few people doing it will go unnoticed.

It's like how Microsoft killed DNT by making it the default. Sites can be like "sure, we can support DNT for the users that are really privacy conscious" but once it's every user the rubber meets the road of ad revenue.