Also Gecko’s development is led by people thinking that it being usable outside of Firefox\Thunderbird is a bad thing. There was a time when Gnome’s browser was based on Gecko, not WebKit. And in general it’s influenced by bad practices.
SerenityOS is an amazing project, of course. To do so much work for something completely disconnected from the wider FOSS ecosystem, and with such results.
So it’s cool that they’ve decided to split off the browser as its own project.