The grey cells marked "don't care" can be on or off, and the resulting pattern will still have the property that it must exist from the start. Some of those said "don't care" cells can also be turned on to form a stable configuration, which is additionally nice.
Hi! At the time of the article's publishing, I had my nickname set to "danielle conway" in the CGoL Discord server, and I guess apgoucher thought that was my full alias. I usually just contribute pseudonymously as either "danielle" or "dani".