Looks like they're using PoS 3.0 on top of Bitcoin 0.19.0... really really nice..
I like the fact that they are active, I mean, looking at the github, lots of nice performance fixes...it's not everyday you see something like this.
Is the same code as Bitcoin Core?
Is a Bitcoin fork?
It's 0.19.0 exactly. Looks like they forked Bitcoin Core and added their code.
I think they made the correct choice, you can see all commits from all contributors, easy to track down, nothing hidden.
By the amount of changes, it looks like a lot of work was put into it.