> same performance as ITER, but in a tenth the size
So instead of being 400 times the volume of a PWR with the same gross power output, they're just 40 times the volume. It's no panacea to the economic challenges facing fusion.
The other way to get high volumetric power density is go with a configuration of higher beta, the ratio of plasma pressure to magnetic pressure (fusion power at a given magnetic field scales as beta^2). Helion isn't using superconductors at all.
REBCO supports stronger magnetic fields, and conveniently, tokamak output scales with the fourth power of magnetic field strength.