Fast quasi-periodic oscillations (QPOs; frequencies of ~20-1840 Hz) have recently been discovered in the ringing tail of giant flares from soft gamma repeaters (SGRs) when the luminosity is of order 10^41-10^41.5 ergs/s. These QPOs must originate from a compact, virtually nonexpanding region inside the star's magnetosphere, emitting with a very moderate degree of beaming. The fastest QPOs imply a luminosity variation of ΔL/Δt~=6×10^43 ergs/s^2, the largest ever observed from a compact source. It exceeds by over an order of magnitude the Cavallo-Fabian-Rees (CFR) luminosity variability limit for a matter-to-radiation conversion efficiency of 100%. Such an extreme variability can be reconciled with the CFR limit if the emitting region is immersed in a magnetic field >~10^15 G at the star surface. This provides independent evidence for the superstrong magnetic fields of magnetars.