It's not source available. It's OpenSource(TM) because of the BSD-2 license.
This is not unheard of. The most famous models are emacs & SQLite. SQLite doesn't accept outside patches, emacs is developed opaquely and only releases are put forward.
You can do this with GPL, too. You put out tarballs of the releases only.
There's a great misconception between Free Software, Open Source, and Open Development (bazaar model). They complement each other, but they are completely independent things.
Addenda: Looks like emacs' Git repo is publicly accessible now, but it's not a requirement for GPL or Free or Open Source software.
It's actually common, many companies develop their products this way. The source is available, you can see the VCS, but you can't participate in the development. That's why I see this as signal that it's going to turn into a company.
Well, technically it is a "company" already as it is registered formally as a non-profit. They have income (sponsors) and paid employees.
To my eye, this change does not appear to be driven by a change in corporate governance or profit motive.
They explain that the change and the timing is driven by two things.
1 - The burden and of processing public contributions has increased with the rise of AI
2 - They need to focus and stabliize the code base in preparation to introduce a public alpha
Those reasons ring true enough for me that I do not need to go looking for other motivations. I do not like this change but I can see why they would.
However many if not most of these companies use "Source Available" licenses which say "Thou shall look, thou shan't compile". This is very different than Open Source license of Ladybird itself.