Ladybird was born from SerenityOS, which is a hobbyist unix-like (or POSIX compliant?) OS that simply aimed to do things “from the ground up”. It just happened that they needed to make a browser, and the response was to make one from scratch.
From there it seemed to have brought a lot of attention organically to the point where it can stand on its own, but originally it was never intended to be a “third browser engine” from its inception.