Agreed the article is unclear. My understanding is they’re using unused capacity on servers that are already performing other tasks. So they’re not really creating a spot market for unutilized servers, so much as for unutilized capacity on the severs they’re already using.

For example if one of their servers is using 50% CPU, they can fill the remaining 50% with video encoding tasks.

But obviously I don’t know any better than you. That’s just my takeaway from the vaguely worded article.