They know exactly what they're doing. They're open-washing the program to get attention. It worked. As I write this, it's the number one YComb story. But, it's in no way, shape, or form open source.
I'm not sure what to call a codebase that only grants you the (implicit) right to not be sued for reading the source and then getting inspired by it, though.
The idea is to avoid accidentally copying code, though TBH i think this is some sort of legend that comes from decades ago and not really practical (nor, as the article mentions, required by law). Writing something similar alone won't make you guilty of anything, otherwise people who worked as programmers wouldn't be able to work at a different company on the same or similar field again ever.