spinifex/docs/compatibility/ec2-api-coverage/intro.md

Spot instances are a mock

Spot Instance Requests are a mock over the on-demand RunInstances path. A request synchronously launches real VMs on the operator's own compute and is then reported active and fulfilled. There is no spot market: no bidding, no price rejection, no interruption and no reclamation, and instances are never taken back.

DescribeSpotPriceHistory is the one operation on that surface deliberately left unimplemented. On owned hardware there is no spot-to-on-demand price differential, so any figure it returned would be invented, and an invented price is worse than an absent one.