The reason for separate systems is that some of them go down. And by "go down", that includes you wanting to deploy a new version. Or restore from backup as the article suggested.

I think it's rather rare to use different services for redundancy and in most cases if you want that redundancy (e.g. to support red-black or blue-green deployments or just to support better availability via failover) I'd usually suggest using the same technology for that purpose.

I don't believe it's ever easier to manage updating two technologies than one.