This is a cluster of commodity machines, a mainframe is one machine.
The mainframe is optimized for reliability and compatibility foremost and does things that are quite unlike most other classes of machines even "high end servers", including RAIM (RAID for memory), and the ability to stop a failed processor and move its checkpointed state to another processor and resume it transparently from software point of view, and generally employing quite hardened circuits and strong error detection and correction throughout the system.
There are some guesses at $500k for this Oxide rack, not sure if that even gets your phone call returned for a low end mainframe with much less compute power and memory installed. High end configurations rumored to be many millions.
This thing is more a competitor to "scale-out" / "cloud" / "webscale" / etc., at least on the hardware front (they seem to do a lot more on software/firmware side than typical such hardware vendors).
