And for those who still can't make much of this MSDS: The compound "Sodium Hydrosulfite" shown in the list of constituents is better known under the name Sodium Dithionite which is a known photographic developer. Since the second developer in reversal process has to develop all the Silver Halide it can find and all Silver Halide has been exposed to strong light, it doesn't matter much that Dithionite fogs and produces poor emulsion speed.
There's a fundamental difference.
The developer reduces AgBr to Ag(0) if the silver halide grain has been exposed to light.
Dithionite reduces AgBr in absence of light.