Earlier quoted context omitted.
It's not enough to guarantee the code has not been copied from somewhere / can be released in the public domain. The only way to have this guarantee is to write it yourself.
I suppose that's true, but that's no different from any other project, no? If I write a contribution to React that wasn't actually mine, it doesn't matter that it's just open source and not being released into the public domain, it's still a violation of the real author's copyright. So if they've chosen a different risk model than essentially every other company doing open source software then that's up to them, but…
There's something specific about the public domain that in many places, you can't just decide to release something in the public domain. I guess that's why they actually accept contribution but require an affidavit dedicating the contribution into the public domain. IIUC an affidavit specifically needs to be witnessed by taker of oaths, which probably ensures that the work can indeed be put in public domain.
Not sure it's the whole story, or even the main reason, or the correct analyze, though.