I am sure a regular dedicated server provider would take a long time to get you the server. Maybe days? Not sure.

I think their point is they provision a "dedicated" server in "minutes."

Does the reduced provisioning time create a huge security nightmare? I don't really see how it does.

The time to provision doesn't really affect the size of the security problem, but it does make it easier to deploy malicious firmware across a fleet of hardware, because everything is behind an API.

It's also much cheaper, since it has per-minute billing rather than per-month billing, and no setup fee. Getting temporary access to 100 Hetzner servers would cost you ~€10,000, because you have to pay a setup fee plus a minimum one month rental for each one, while it looks like it'd only cost you ~€100 to get 100x OnMetal servers for an hour. If someone stands to benefit enough, you'd be screwed in either situation, but it lowers the cost of opportunistically sowing some malware.

Most hosters or clouds already provide you servers in seconds / minutes.