Are distributed distributions like NixOS able to replace docker in terms of autoscaling?
Autoscaling has nothing to do with NixOS or Docker. This is just the feature of cloud backed applications.

However it is true that the best results are achievable using immutable infrastructure, but you do not need neither of these tools to achieve such. Instead you can build your own AMI (using Packer) and then use AWS CodeDeploy to deploy your application on each of the instances.

These tools are orthogonal, so you can use them, but there is no problem with using autoscaling without them.