I am the author of that add-on ... what would you do in my shoes ? - provide an XPI (that will only benefit the few since the side-loading process is made awkwarder at every point release) ? - fight it ? If so, on what grounds and how ? - something else ?
Also, anything that gets (or forces) the actors behind this to issue a takedown request against GitHub is good, because then there's a chance GitHub will tell them to pound sand (sending a strong signal to others that unconditional compliance with such demands may not be the only, or best, course of action) or actively fight it.