It would be helpful if the author specified what the price difference actually was here at the end of the day - their initial cost was $3,500/mo but they don't mention how much they pay now after changing instance types.
I would guess he more or less halved the price if you suppose at any minute 70% of the pods are idle, but they still consume > 0 resources, and the price per vcpu didn't change, like he said. Then you also have the overhead of running the Kubernetes processes which is cut to about 1/100th lowering the price further.
Not real numbers, but gives you an idea.
Let's not forget that you'd also have to pay at least 3 (8 hours per shift, not accounting for weekends) people to ensure 24/7 service availability through those 15 Hetzner nodes.
AWS/GCP/Azure et al make it so you generally don't need to worry about the infrastructure part of the problem. Everything else still applies though.
This is true if you run your own datacenter, but dedicated server providers typically monitor their servers and immediately intervene if they detect any connectivity/network issue (which is outside our control even if we use cloud providers like AWS).
For individual server failure (disk issue, etc), how you handle failure condition on dedicated servers is not much different than handling failure on cloud servers (remove the bad server from the cluster, re-image a new server and join it to the cluster). The main difference is provisioning new physical server is not instant, so you'll need to plan ahead and either have some spares or slightly over-provision your cluster (so you can take down a few nodes without degrading your service). You can do this automatically or manually, not much different than using cloud servers.
Using dedicated servers is not as scary as what people thought it would be.