Public domain may not be the default status of a published work, at least in the USA.
Not may not, it is not. Copyright is automatic. The terms of use you assign, either by agreeing to a ToS or an explicit license, are largely separate.
This means that code is essentially untouchable in its whole form. Without a declaration, the originator owns it, IMHO.
That's not just your opinion, that's how it is. You own your work unless you've assigned ownership to someone else, or done as work for hire, under contract, etc. That's licensed or not. You continue to "own" your work, even if you eventually license it permissively. That means you can do a fork of your own work that's completely closed too, if you like, so long as you don't run afoul of any previous or interim contributors' work and licensing.
Don't confuse that with the ability to retroactively revoke the terms of a permissive license though. You can't just yank GPL licensed code back from people that are using it. The license is designed that way.