A container though should be immutable and ideally shouldn't have changes made to it. If the container were to die, it'd revert back to the old version? It looks to me like these seamless upgrades would be an anti pattern to containers.
With ingress you'd have a load balancer in front or have it routed in the network layer using BGP.
I think the parent was talking more about the fact that at some point, you have a component that should be available as much as possible. In the case you mention, that would be the load balancer. Being able to upgrade it in place might be easier than other ways.
You would need more than one to do a rolling restart. Alternatively to do it with one instance of a software load balancer is a bit more work, spin another instance up and update DNS. Wait for traffic to the old one to die as TTLs expire, then decommission.
But I agree it isn't as easy as a in place upgrade.
With ingress you'd have a load balancer in front or have it routed in the network layer using BGP.