Yep fair point I added that to the docs bug report.
This setting is only ignored during a graceful node restart (i.e. kubectl delete pod). If the node is not expected to come back after a restart, or it falls out of the cluster unexpectedly, then index.unassigned.node_left.delayed_timeout will apply.
The value in use is under ECK's control since it's ECK that calls the put-shutdown API. I don't know if it's configurable there, you would need to ask some ECK experts about that.
Can you share a little more context? Why do you want to force faster replica allocations (and a potential rebalancing storm) when doing a graceful node restart?