I wonder what they wrote it in. I'd guess C++, Java or Erlang, or a combination of those.
Chances are it's IPVS[1], maybe with some patching. Why would one reinvent the wheel when Linux already lets you do that. In that case it would be C since it's implemented in the Linux kernel. [1] https://en.wikipedia.org/wiki/IP_Virtual_Server
This appears as Layer 4 load balancing, IPVS is more of Layer 3.
So the person who was voted down by mentioning HAProxy might not be too far off. It could be implemented through HAProxy + TPROXY enabled in the kernel[1]. Then just make sure that default gateway configured on targets routes back to the load balancer or it is the load balancer.
[1] https://www.loadbalancer.org/blog/configure-haproxy-with-tpr...