Again, I refer you back to Mees and James Alan.
Chemical reactions are often slow. They can be sped up considerably by removing one of the products from the mixture. So, Metol is slow to oxidize, but add sulfite and it speeds up the oxidation. The same is true of HQ, it is slow to oxidize but it speeds up with sulfite. This is NOT superadditivity, but merely shifting the balance of a chemical equation. (if you substitute the word 'develop' in the above for the word 'oxidize' then you have the nature of the reaction in a developer).
In Mees and James they offer proof by using a different oxidant and demonstrating the same effect in the presence and absence of sulfite as we see in a developer. They show that this reaction is general, not just specific to a developer solution.
Mixing Metol and HQ together gives a synergistic reaction (superadditivity) that is different than the effect of sulfite. Sulfite speeds up this reaction as well, but by changing the reaction constant.
A + B * K = C where K is a constant. A and B are an oxidant and reductant respectively and C is the oxidized B (oxidized reductant).
A + B + D * K * KK = C + D = E where K is the original constant and KK is the constant supplied by removing C from the right of the equation by the action of D. Therefore, in the second reaction D = sulfite and C = Quinone and E = HQ monosulfonate.
This equation is a simple, non-balanced version of the redox reaction of HQ in the presence of silver halide and with and without sulfite. It is not even written in classic chemical terms. In those terms the "=" is changed to an arrow "->" or a double arrow (one in reverse "<-") with "K" above the arrow and "KK" or K1" below the reverse arrow plus other modifications. These cannot easily be shown without graphics.