Couldn't someone just modify it to remove the key check?
The creation of a keygen, which doesn't require any modifications to the original software, is mainly for the challenge of doing so, and now that XP is out of support, historical/archival purposes.
All kidding aside, this looks like a fork or reimplementation for the tool that got popular recently, based on a blog post from a few years back.
The music is delightful https://www.youtube.com/watch?v=KE2uyF4PzNI
Here's some more details:
https://sabah.forumotion.com/t333-all-you-need-to-know-about...
The key phrase is at the end: "Therefore, the complexity of computing the private key k is O(2^31)"