I've looked in a bit more detail and it seems to attach the nodes to the ELB used by the AutoScaling group, while I am attaching them to the group itself, which indirectly adds them to the load balancer.

I'm curious how does that tool handle the scaling of the group, since the nodes are actually outside the group and can't contribute to group-wide metrics like average CPU usage, often used for scaling out.

I didn't know about that one, indeed, it seems pretty similar.