Author here! Spot instance pricing is better than on-demand, but it doesn't include data transfer, and it's still more expensive than on-prem/Hetzner/etc. Data transfer costs exceed the cost of the instance itself if you're transferring many TB off AWS.

For of our more popular AWS instance types I use – a c5a.24xlarge, used for comparison in the post – the cheapest spot price over the past month in us-east-1 was $1.69. That's still $1233.70/mo: above on-prem, colo, or Hetzner pricing. Data transfer is still extremely expensive.

That said, for bursty loads that can't be smoothed with a queue, spot instances (or just normal EC2 instances) do make sense! I use them all the time for my computational biology company.