Does "external" contributors effectively mean drive-by contributors? I assume there's still more than a single developer, and it's somehow possible to be admitted as a member of the project if you have some existing public reputation.
Website says: "Ladybird is open source, developed by a small team in the open. You can help by filing bug reports, creating reductions, testing websites, reporting security issues, and sharing technical feedback." [1]
I bet if you contribute in those various other ways for a while, you can probably eventually submit patches. Not as exciting as being able to jump right in, for sure. But it sounds like there's a way in, and I guess you can carry your own patches in a downstream fork in the meantime.