If you flip a coin and generate a bitcoin address from it, there's two possible keys. Two coin flips, four possibilities. After 80 coin flips, you've got more possible keys than a regular computer can loop through in a lifetime. After some 200 coin flips, the amount of energy to check all of them (if you're guessing the generated private key for this address) exceeds what the sun outputs in a year iirc (or maybe all sunlight that hits the earth — either way, you get the idea: incomputable with contemporary technology). Exponentials are a bitch or a boon, depending on what you're trying to achieve.
Per another comment <https://news.ycombinator.com/item?id=38467969>, existing bitcoin addresses is what they found being generated. There is physically no way that's a coincidence.
Perhaps it live queries the web, that's an alternative explanation you could prove if the authors are wrong (science is a set of testable theories, after all). The simplest explanation, given what we know of how this tech works, is that it's training data.