Because there are only like 3 browser engines: Chrome’s Blink, Firefox’s Gecko and Apple‘s WebKit. And while they are all open source, KHTML, the last independent browser engine got discontinued last year and hasn’t been actively developed since 2016.
There’s need in the space for an unaffiliated engine. Google’s share is far too high for a healthy market (roughly 75%), WebKit never got big outside of Safari (although there are a few like Gnome Web, there’s no up to date WebKit based browser on Windows) and Gecko has its own problems (like lack of HEVC support).
So, in my book, this is exciting news. Sure it‘ll take a while to mature and it is up against software giants but it‘s something because Mozilla doesn’t seem to have a working strategy to fight against Google‘s monopoly and Apple doesn’t have to.