It's a difference only insofar that in many jurisdictions their claim that it's public domain has no legal value. If it was truly public domain (e.g. if the authors were long dead) it would be open source. But far from all places allow you to arbitrarily put things in the public domain.

I'm a bit puzzled why SQLite doesn't solve this trivial issue by claiming the code is CC0-licensed. CC0 is made just for that: a very wordy way to make it as close to public domain as possible in each jurisdiction.

On the other hand, hobbyists won't care. As long as you trust them in their intention to have it open source they won't sue you for infringement either. And if as a company you need more assurance than "it's public domain" they are so nice to sell you a fancy legally-satisfying piece of paper for an undisclosed price. It's a subtle but clever way to get income from users with too much money