"Real" mainframes have RAS (Reliability, Availability, Servicing) features such as hotswapping for all hardware components and automated HA/workload migration across physical racks. They can also do SSI (single system image), i.e. run a single workload across physical nodes/racks as if it was just multiple 'cores' in a single shared-memory computer. Oxide computers will probably end up doing at least some of this (namely workload migration across racks for HA) but saying that it can comprehensively replace mainframe hardware as-is is a bit of a stretch. In terms of existing hardware it's closer to a midrange computer.
The Oxide and Friends podcast had an episode on virtualizing time, specifically for the purpose of live-migrating a container from rack to rack without the VM being aware, and allowing operators to take the rack offline on their schedule. Otherwise, apparently, you end up having
to leave racks running because you cannot evacuate all of the containers currently running on it. (e.g. perhaps your contracts or SLAs are such that you cannot afford even the few seconds of downtime a shut-down-here-and-spin-up-elsewhere would cause)
I believe the episode name was "Virtualizing Time"
