The article does not explain how they entered it into the public domain. One cannot just declare something is public domain. Public domain is a special circumstance.
Of course you can. Lots of people do this. There are whole websites that collect and distribute public domain photos, music, artwork, etc.
I'm actually very curious now what led you to the line of thinking that it isn't possible?
Depends on the jurisdiction—e.g. Germany[1] is known to be particularly problematic. But in this particular case, the model has been released under CC0[2], which has a fallback permissive license alongside its public domain dedication, specifically to avoid this problem. That’s why the CC0 is a thousand words long instead of a couple of sentences. Most sites hosting “public domain” works also use the CC0 or something similar.
[1] https://opensource.stackexchange.com/q/9871
[2] https://creativecommons.org/publicdomain/zero/1.0/legalcode....
So if I made Benchy longer and turned it into a rendition of the Titanic, I can't claim that it's 100% mine and no one else had a hand in making it under those rules.
This is why the CC0 license exists, to attempt to provide a more legally sound equivalent to plain public domain (https://creativecommons.org/public-domain/cc0/).
The public domain is the set of all works in which no copyright subsists. Copyright automatically subsists in a work from its creation until it expires. Copyright is a property right. All property must have an owner.
Therefore a work cannot enter the public domain unless the copyright subsisting in it has expired.
(This is from the perspective of the law of England and Wales, at least. Other jurisdictions have similar legal axioms that produce the same result, though)