It was written for an older version of cs-cart. Also AfterPay has rebranded and they are a stickler for compliance before issuing live credentials. I was able to make it work with some modification though in 4.11. You basically need to replace all the brand assets and then update the api request to include platform info to satisfy AfterPay, and I think one of the functions the module uses was depreciated, either by cs-cart or PHP, can't recall exactly, but it was easy to fix. Hardest part was dealing with AfterPay compliance hound.
TLDR: Don't expect it to work out of the box, but you can get it to work and it's probably worth it rather than make your own from scratch. When I asked about it when I got it, the company said it hasn't been maintained for ages so they weren't really in position to do anything but were happy to refund me if I didn't want to keep it.