using on-demand for latency insensitive work, especially when you’re also very cost sensitive, isn’t the right choice. spot instances will get you somewhere in the realm of the hetzner/on-prem numbers.
Personally, when I estimate the total cost of ownership of scientific cloud computing versus on prem (for extremely large-scale science with both significant server, storage, and bandwidth requirements) the cloud ends up winning for a number of reasons. I've seen a lot of academics who disagree but then I find out they use their grad students to manage their clusters.
Also, AWS is notoriously easy to undercut with on-prem hardware, especially if your budget is large and your uptime requirements aren't - you'll save a few hundred thousand a year alone by not having to hire expert engineers for on-call duty and extreme reliability.
Using the estimate from the article, spot instances are still over 8x more expensive than on-prem for scientific computing.